Many bioactive peptides can be derived from a wide range of protein‐rich resources, such as bovine and human milk, fish of various species, egg, meat, soybean, rice, sunflower and different cereals. These potential peptides can be used as functional ingredients in the prevention of diseases (Hartmann and Meisel 2007 ; Erdmann and others 2008 ; Lafarga and Hayes 2016 ). Recently, the knowledge of diverse bioactive compounds of seaweed have opened up potential opportunities for development of pharmaceuticals (Aneiros and Garateix 2004 ; Martinez‐Augustin and others 2014 ). As a result, seaweeds having medicinal applications, and their known functional therapeutic properties have been screened (Jo and others 2016 ). This review centers on therapeutic effects of seaweed protein–derived peptides, particularly that have antihypertensive (ACE inhibition), antioxidant and antidiabetic effects. The isolation methods of peptides, current status and future prospects of peptides are also discussed.

Food‐derived bioactive peptides commonly contain 2 to 20 amino acids and they are inactive when encrypted in their native protein structure. They need to be released by protein degradation in either of the following ways to perform their specific roles (Moller and others 2008 ; Udenigwe 2014 ; Capriotti and others 2015 ): (i) endogenous and exogenous microbial enzymatic action (fermentation of food); (ii) food processing and gastrointestinal digestion of dietary proteins ( in vitro or in vivo digestion) using various proteolytic enzymes, such as pepsin, trypsin, alcalase, and pancreatin (Erdmann and others 2008 , DiBernardini and others 2011 , Agyei and Danquah 2012 , Capriotti and others 2015 ). Depending on their amino acid sequence, these peptides can accomplish wide‐range of activities (Figure 1 ) such as antioxidant, cholesterol‐lowering capacity, enhancing mineral absorption, immunomodulatory, antimicrobial, antithrombotic, antiobesity & antidiabetics, and blood pressure‐lowering (Hartmann and Meisel 2007 ; Erdmann and others 2008 ; Capriotti and others 2015 ).

The advent of the modern drug era has started during World War I with the introduction of opiate morphine and the cyclic peptide penicillin and this was quickly followed by the introduction of the (poly) peptide insulin in early 1920s with the emerging of new disease treatment standards (Uhliga and others 2014 ). Since then, the number of peptide drugs entering to clinical trials is increasing progressively: it was 1.2/y in the 1970s, 4.6 in the 1980s, 9.7 in the 1990s and 16.8 in the 2000s (Danquah and Agyei 2012 ). Currently, nearly 140 peptide drugs are in clinical trials, about 500 are in preclinical development (Fosgerau and Hoffmann 2015 ) and more than 60 are in a market approved by U.S. Food and Drug Administration (FDA) (Thundimadathil 2012 ; Fosgerau and Hoffmann 2015 ; Lau and Dunn 2016 ), as of 2016 (Lau and Dunn 2016 ) generating more than $13 billion in annual sales. The tremendous increase of interest in peptide‐based therapeutics is due to the advances in drug delivery systems in the last 10 y. Certain peptides can pass through cell membranes and are used as carriers for targeted drug delivery. The use of peptides is also growing in medical diagnostics, nutraceuticals, antimicrobial, and cosmetics.

Food metabolism in the body releases specific molecular functional groups or their derivatives. These specific groups are involved in the therapeutic activities of functional foods and nutraceuticals (Agyei and Danquah 2012 ). Dietary bioactive peptides are among the major groups that have received huge consumer interest (Barbé and others 2014 ) and gained an increased application on health‐promoting functional foods and in different therapeutics (Thundimadathil 2012 ), due to desirable physiological responses they trigger (Agyei and others 2016 ). Nature harbors variety of biologically active peptides that serves as the most important sources for drug discovery (Uhliga and others 2014 ). About, 7000 naturally occurring peptides have been identified. These peptides play vital roles in human biological activity and in effective signaling through binding to specific cell surface receptors, where they trigger intracellular effects. In addition to their pharmacological profiles and intrinsic properties, they have remarkable safety, tolerability, and efficacy in humans. Hence, they are excellent starting point for the design of novel therapeutic drugs (Fosgerau and Hoffmann 2015 ).

Foods are sources of other promising bioactive substances that have potential impact on human health in addition to common nutritional components (Barbé and others 2014 ). Recently, consumer knowledge has substantially changed concerning the relationship between diet and health. The role of diet, beyond the basic nutrition, has gained increased acknowledgment. Food, not only, is intended to satisfy hunger and provide the essential nutrients, but also contributes to prevention of nutrition‐associated diseases, reduce health risks, and improve human well‐being (Betoret and others 2011 ; Gupta and Abu‐Ghannam 2011 ; Harnedy and FitzGerald 2011 ). Cognizance of this, the interest toward functional foods and nutraceuticals has changed significantly. Nutraceuticals, which combines nutrition and pharmaceuticals together, and functional foods are food components or food additives that provide beneficial health effects to the body by reducing the risk of certain disease or any health concerns (Admassu and others 2015 ).

The hydrolysis products, bioactive peptides, need to be further concentrated and fractionated in to their different molecular weight cut off (MWCO) to isolate peptide of interest for a particular bioactivity. Usually an ultrafiltration membrane and gel permeation chromatography are the usual methods used to fractionate and separate peptides from bulk hydrolyzed mixtures into fractions having various molecular weights (Figure 2 ) (Harnedy and FitzGerald 2011 ; Cheung and others 2015 ). Ion exchange, affinity chromatography and high performance liquid chromatography (HPLC) are also the most important chromatographic approaches (Figure 2 ) widely used for purification and enrichment of bioactive components (Agyei and Danquah 2011 ; Harnedy and FitzGerald 2011 ; Li‐Chan 2015 ). Qu and others ( 2015 ) have reported a novel bioreactor technology, which coupled enzymatic hydrolysis and membrane separation (CEH‐MS) in a multistep recycling system and fractionate the hydrolysates according to ranges of their molecular weight. This system utilizes the benefits of biochemical engineering and membrane separation techniques to have efficient separation of target components. Following the chromatographic approaches, liquid chromatography–mass spectrophotometry (LC–MS) and mass–mass spectrophotometry (MS–MS) are performed for the identification of the amino acid sequence of the isolated peptides to characterize their molecular structures and masses (Samarakoon and Jeon 2012 ), and then bioactivity is validated by testing chemically synthesized pure peptides.

After the extraction, proteins are subjected to hydrolysis in order to release functional peptide fragments with their specific bioactivities. Among the different hydrolysis methods, enzymatic hydrolysis is favored by nutraceutical and pharmaceutical industries, which avoids harsh chemical and physical treatments. More importantly, functional properties and nutritional values can also be preserved (Cheung and others 2015 ; Li‐Chan 2015 ). As it is noted previously, the bioactive peptides from the inner cellular seaweed like other protein sources, can be obtained basically by hydrolysis using digestive enzymes, proteolytic enzymes or proteolytic microorganisms during fermentation (Samarakoon and Jeon 2012 ). Table 1 and 2 shows that enzymes, including pepsin, trypsin, papain, chymotrypsin, alcalase, fungal proteases and Flavourzyme, and Corolase PP have been used more commonly for hydrolysis of seaweed proteins in producing ACE inhibitory and antioxidant peptides. In this enzymatic hydrolysis process, the encrypted peptides can be released to play their specific role. These proteolytic hydrolyzing enzymes can work either separately or in a serial combination of them for the production of bioactive peptides, which range from 2 to 20 amino acid compositions.

Ultrasound pretreatment for protein extraction from Ascophyllum nodosum with a probe type of ultrasound equipment with 750 W capacity and 20 kHz frequency; amplitude levels of 22.8 and 68.4 μm was employed and compared with acid or alkaline treatment alone (Kadam and others 2016 ). This treatment has increased the protein extraction yield by 540% and 27%, respectively and reduced the processing time from 60 to 10 min (Kadam and others 2016 ). In addition, Qu and others ( 2013a ) suggested that the use of 2 alternating counter‐current frequencies (15 and 20 kHz) improves protein extraction on Porphyra yezoensis , and the yield was increased by 50%, with a reduction of the extraction processing time by 18% when compared with monofrequency ultrasound‐assisted extraction. Ultrasound‐assisted solvent extraction to recover high‐added value compounds from the microalgae Nannochloropsis spp. was reported as a promising tool. The extraction yields for ultrasound‐assisted method was 2 times higher than that of conventional water extraction and processing time had a significant influence on antioxidant compounds recovery (Parniakov and others 2015 ).

The use polysaccharidase enzymes such as Viscozyme®, Cellulase(Celluclast®), xylanase (Shearzyme®), κ‐carrageenase, β‐agarase, Ultraflo® (β‐glucanase) for cell disruption prior to protein extraction appears to increase protein yield (Fleurence 1999 ; Bleakley and Hayes 2017 ). It has been reported that a simultaneous application of Cellulase and xylanase on Palmaria palmata seaweed improved protein extraction efficiency with a factor of 2.5 to 3.5 depending on the concentration of enzymes and harvesting season. Particularly, this treatment has increased the R‐Phycoerythrin protein by 20% to 23% than without enzyme extraction method (Joubert and Fleurence 2007 ). Another study reported that xylanase increased extracted protein yield from P. palmata by approximately 67% (Harnedy and FitzGerald 2013a ). However, the enzyme: substrate concentration required (48.0 × 103 units/100 g) was high making it less feasible for its applicability in industrial scale.

The first step in the production of bioactive peptides is identifying appropriate protein sources and selecting extraction methods (Cheung and others 2015 ; Li‐Chan 2015 ). As mentioned above, the cell wall polysaccharides and their structural complexity and rigidity resulted in inadequate extraction and utilization of the intracellular bioactive proteins from seaweeds (Wang and others 2010a , 2010b ), and this in turn results in low extraction yield and limited the application of seaweed proteins and peptides. To facilitate the disruption of the cell wall and gain access to the seaweed protein, grinding in liquid nitrogen approach was used. However, this approach is not cost effective at industrial scale and not fully efficient in the degradation of cell wall. The efficiency of classic extraction methods is substantially hindered by high viscosity and anionic bonding of these cell‐wall polysaccharides with glycoproteins and their presence in the cell wall in large quantities (Dumay and others 2013 ). New approaches have recently been available to improve protein extraction efficiencies to obtain desirable functional properties. These include: microwave‐assisted extraction, supercritical fluid extraction, pressurized solvent extraction, ultrasound‐assisted extraction, pulsed electric field‐assisted extraction and enzyme‐assisted extraction (Wijesinghe and Jeon 2012 ; Cheung and others 2015 ). These technologies allow the production and isolation of bioactive substances of interest with functional properties through distraction of the cell wall polysaccharides (Ngo and others 2012 ).

The structure and biological properties of seaweed proteins are still poorly documented. However, it has been reported that most seaweed bioactive constituents including protein are intracellular under a highly rigid and structural complex algal cell wall, which is a major obstacle to the efficient extraction and digestibility of protein fractions (Wang and others 2010a , 2010b ; Harnedy and FitzGerald 2011 ; Fleurence and others 2012 ). This is indicating that the nature of seaweed protein is highly cohesive with polysaccharides.

Bioactivities of Seaweed Protein‐Derived Peptides

As a natural origin, protein‐derived peptides can be used as persuasive alternatives in the pharmaceutical and biotechnological industries for chemosynthetic drug candidates. This is due to an ever‐increasing consumer awareness of safety and the low price (Jao and others 2015), the discovery of the bioregulatory role of different endogenous peptides in the organism, and the understanding of the molecular mechanism of actions of bioactive peptides on a specific cellular targets has augmented (Aneiros and Garateix 2004). Due to their extraordinary diversity (Harnedy and FitzGerald 2011), seaweeds are being explored for various commercial food, agricultural & horticultural, pharmaceutical, cosmetic, and bioenergy applications (Beaulieu and others 2016). They are potential reservoirs of novel biologically active components; more specifically, they are rich sources of proteins and amino acids and thus seaweeds can be used as a potential starting materials in the production of (Harnedy and FitzGerald 2011) bioactive peptides with a wide range of bioactivities, depending on their structure, composition, and their amino acids sequence (Ngo and others 2012; Daud and others 2016).

Antihypertensive peptides Cardiovascular diseases (CVD), including heart diseases and stroke are major syndromes that affect the human circulatory system (Collins and others 2016). The major risk factor for CVD is hypertension, which is the elevation of arterial blood pressure (Kim and Wijesekara 2010; Fitzgerald and others 2014; Cheung and others 2015; Collins and others 2016) and it affects 15% to 20% of the global population (Ko and others 2011). Renin and angiotensin converting enzyme (ACE) are the 2 key enzymes in renin‐angiotensin system (RAS), the endocrine system that regulates peripheral blood pressure (Harnedy and FitzGerald 2013b; Cheung and others 2015). ACE is a multifunctional enzyme which plays a vital role in the regulation of blood pressure (Shi and others 2004; Collins and others 2016) and bradykinin degradation, a blood pressure lowering peptide in the kallikrein–kinin system (Shi and others 2004; Liu and others 2010). When blood flow to the kidneys is low, renin is produced, and converts angiotensinogen to inactive angiotensin‐I (Decapeptide = Asp‐Arg‐Val‐Tyr‐Ile‐His‐Pro‐Phe‐His‐Leu) (Figure 3), which is converted to the potent vasoconstrictor angiotensin‐II (octapeptide = Asp‐Arg‐Val‐Tyr‐Ile‐His‐Pro‐Phe) by ACE. In addition, angiotensin‐II is involved in the release of Na‐retaining steroid, aldosterone, from the adrenal cortex, resulting in raised blood pressure (Shi and others 2004; Torruco‐Uco and others 2009; Harnedy and FitzGerald 2013b; Cheung and others 2015). Figure 3 Open in figure viewer PowerPoint 2004 Role of ACE blood pressure regulation in renin—angiotensin–aldosterone system and the inhibitory action of ACE inhibitors (Shi and others). One of the key therapeutic approaches in the management of hypertension is inhibition of ACE (Wijesekara and Kim 2010). ACE inhibitors block conversion of angiotensin I into angiotensin II, resulting in blood vessel relaxation and decreased blood pressure (Cheung and others 2015). Pharmaceutical companies have marketed various ACE inhibitors for the management of hypertension through the reduction of angiotensin II concentration and hence reduction of blood pressure. But these drugs have various undesirable side effects, suggesting the need for natural food derived inhibitors of ACE for controlling hypertension, with minimal adverse side effects (Torruco‐Uco and others 2009). Food protein‐derived peptides are considered to be milder, safer, and easily absorbed when compared with synthetic drugs (Liu and others 2010; Cheung and others 2015). The peptides with ACE inhibitory activity have been isolated from a number of animal and vegetable sources of protein hydrolysates (Otte and others 2007; Je and others 2009; Quist and others 2009; Liu and others 2010; Qu and others 2013b; Malomo and others 2015; Zhou and others 2015; Daud and others 2016). Within the food industry, marine organisms are becoming the focus of research due to their numerous positive health effects. Several studies have described a number of marine‐derived bioactive peptides with potent ACE inhibitory activity (Wijesekara and Kim 2010). More specifically, increasing consideration has been given to macro algae (seaweed) as a potential source owing to their high protein content (Walker and others 2009; Harnedy and FitzGerald 2011). Studies have reported that bioactive peptides and protein hydrolysates of seaweed can inhibit ACE and renin enzymes (Table 1) (Shi and others 2004; Sheih and others 2009a; Fitzgerald and others 2012; Ko and others 2012; Harnedy and FitzGerald 2013b; Qu and others 2015; Furuta and others 2016). Fitzgerald and others (2012) has identified 11 (IRLIIVLMPILMA, ILMA, MNEIVALMI, ILMA, LMAASWAIY, ILPSILVPLV, PSIL, LVPLVGLV, PLVGLVFPAI, VFPAIAM, FPAI, and PAIA) renin inhibitory bioactive peptides from papain hydrolysate of Palmaria palmata. All elucidated peptides were chemically synthesized and assayed for their renin inhibitory abilities and 1 tridecopeptide [IRLIIVLMPILMA (Arg‐Leu‐Ile‐Ile‐Val‐Leu‐Met‐Pro‐Ile‐Leu‐Met‐Ala)] has inhibited renin by 50% at a concentration of 3.344 × 103 μM (±0.31). This peptide was well comparable with the positive control (Z‐Arg‐Arg‐Pro‐Phe‐His‐Sta‐Ile‐ His‐Lys‐(Boc)‐OMe), which inhibited renin by 50% at a concentration of 1 × 103 μM (Fitzgerald and others 2012). In another study, 9 ACE inhibitory peptides (YRD, AGGEY, VYRT, VDHY, IKGHY, LKNPG, LDY, LRY, FEQDWAS) were isolated from dulse (Palmaria palmata) hydrolysate and the synthetic peptide LRY (IC 50 : 0.044 μmol) has shown remarkably high ACE inhibitory activity (Furuta and others 2016). ACE inhibitory activity of the hydrolysates generated by Alcalase, Corolase PP and Flavourzyme enzymes from aqueous, alkaline and the mixture of the 2 solvent protein extracts of P. palmata was reported (Harnedy and FitzGerald 2013b). Significantly higher ACE inhibitory activity was observed with IC 50 value of: 0.19, >2, and 0.33 mg/mL for Alcalase, Corolase PP, and Flavourzyme hydrolysates, respectively from aqueous protein extracts, 0.41, >2, and 0.78 mg/mL for Alcalase, Corolase PP, and Flavourzyme hydrolysates, respectively from alkaline protein extracts and 0.28, >2, and 0.34 mg/mL for Alcalase, Corolase PP, and Flavourzyme hydrolysates, respectively from alkaline and aqueous mixture protein extracts. The IC 50 values of 15 and 177 nM were obtained for the synthetic ACE inhibitory drugs captopril and enalapril, respectively. ACE and renin inhibitory peptides derived from seaweed protein including their source (seaweed species), method(s) used to produce or type of enzymes used for hydrolysis, and IC 50 values of these peptides reported by many researchers are summarized in Table 1. Although the precise inhibition mechanism of ACE‐inhibitory peptides is not clearly known, the in vitro inhibition mode of peptides is evaluated by performing Line weaver–Burk plots (Shi and others 2004; He and others 2013) and the potency is stated by their IC 50 value, which is the concentration of the inhibitory peptides required to inhibit 50% of ACE activity (He and others 2013). Most studies mentioned that ACE inhibitory peptides are competitive inhibitors; however, noncompetitive or uncompetitive mode of inhibition has also been reported. For example, a potent ACE inhibitory peptide with amino acid sequence, Val–Glu–Gly–Tyr (MW: 467.2 Da, IC 50 value of 128.4 μM) from green microalgae (Chlorella ellipsoidea) was isolated and the Lineweaver–Burk plots suggest competitive inhibition (Ko and others 2012). Furthermore, the activity of this purified peptide against ACE was not affected when incubated under simulated gastrointestinal conditions, signifying that Val–Glu–Gly–Tyr is stable against gastrointestinal enzymes of pepsin, trypsin and chymotrypsin. The antihypertensive effect of the purified peptide in spontaneously hypertensive rats (SHR) was also evaluated by measuring the change of systolic blood pressure (SBP) for 0, 2, 4, 6, and 8 h after single oral administration with a dose of 10 mg/kg body weight. The peptide reduced systolic blood pressure significantly and the activities were maintained for 6 h. The maximum decrements in SBP of the peptide and captopril (positive control) were 22.8 and 35.4 mmHG at 4 h, respectively (Ko and others 2012), a reason for optimism that peptides could be used as alternatives to synthetic ACE inhibitors. Another study revealed that noncompetitive pattern of ACE inhibition was obtained for the peptide (Val‐Glu‐Cys‐Tyr‐Gly‐Pro‐Asn‐Arg‐Pro‐Gln‐Phe) isolated from algae protein waste (Sheih and others 2009a). This purified peptide was subjected to incubation at temperature range of 40 to 100 °C and pH range 2 to 10 to investigate the pH and heat stability. Interestingly, the residual activity showed that the purified peptide retained its ACE inhibitory activity. The inhibitory activity of the purified peptide was also not affected by the gastrointestinal enzyme digestion of pepsin and pancreatin (Sheih and others 2009a). Seven ACE inhibitory dipeptides with amino acid sequence (Val‐Tyr, Ile‐Tyr, Ala‐Trp, Phe‐Tyr, Val‐Trp, Ile‐Trp, and Leu‐Trp) were isolated from Wakame (Undaria pinnatifida) by (Sato and others 2002). They exhibited different inhibition mode against ACE activity. Val‐Tyr and Val‐Trp showed a competitive inhibition; Ile‐Tyr and Leu‐Trp showed a noncompetitive inhibition, and the other peptides (Ala‐Trp, Phe‐Tyr, and Ile‐Trp) showed uncompetitive inhibition. For the evaluation against spontaneously hypertensive rats (SHRs), Val‐Tyr, Ile‐Tyr, Phe‐Tyr, and Ile‐Trp significantly reduced the blood pressure when treated with a single oral administration dose of 1 mg/kg mouse (Sato and others 2002). In a report by Suetsuna and others (2004), 10 dipeptides (Tyr‐His, Lys‐Trp, Lys‐Tyr, Lys‐ Phe, Phe‐Tyr, Val‐Trp, Val‐Phe, Ile‐Tyr, Ile‐Trp, and Val‐Tyr) isolated from the hot water extract of U. pinnatifida decreased blood pressure in SHR.

Antioxidant peptides The oxidation of lipids and proteins of food products (Dong and others 2016) during processing or storage by reactive oxygen species (ROS), such as superoxide anion radical (O 2 −•), hydroxyl radical (·OH), hydrogen peroxide (H 2 O 2 ), singlet oxygen (1O 2 ), and Peroxyl radical (•OOR) (Li and others 2009; Gu and others 2014), is the major reason for food deterioration that would reduce consumer acceptability of food due to undesirable changes of quality and the possible production of toxic compounds (Balti and others 2011; Ganesan and others 2011; Dong and others 2016). Consuming these potentially toxic products may trigger various human chronic diseases, including cancer, arteriosclerosis, aging (Ganesan and others 2011; Dong and others 2016) diabetes mellitus, inflammation, coronary heart diseases, and neurological disorders, such as Alzheimer's disease (Kim and Wijesekara 2010; Balti and others 2011). Therefore, to prevent food products from such deteriorations and protect consumers against the serious diseases, 1 key strategy is inhibition of lipid peroxidation occurring in the living body and food products by using antioxidant substances or preservatives (Choe and Min 2009; Balti and others 2011). Antioxidants or preservatives are chemical components in biological materials, relatively found in low concentrations that prolong the shelf life of food by delaying or inhibiting oxidation of a substrate in the food (Balboa and others 2013; Power and others 2013). Bioactive peptides are the most commonly occurring antioxidant substances in food. Synthetic substances, for example, butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propylgallate, TBHQ (tert‐butyl hydroquinone) have better antioxidant activity and retarding effects of oxidation than those of the natural antioxidants. However, the use of these chemical antioxidants needs strict control due to their potential health risks and toxicity (Heo and others 2005; Balti and others 2011; Gu and others 2014). Moreover, interest of consumers toward minimally processed foods and natural preservatives is pushing the researchers and food manufacturing industries to look for safe food antioxidants from various natural sources (Kim and others 2001; Heo and others 2005; Kim and Wijesekara 2010; Gupta and Abu‐Ghannam 2011). In view of this, food protein hydrolysates and bioactive peptides are given priority (Balti and others 2011). For such and other diverse health‐promoting properties, bioactive peptides from macro algae are being investigated increasingly with remarkable progresses (Cian and others 2012a, 2012b; Harnedy and FitzGerald 2013b; Beaulieu and others 2016, 2013). Harnedy and FitzGerald (2013b) have investigated the antioxidant potential of Palmaria palmata for aqueous, alkaline and mixture of aqueous and alkaline protein fraction hydrolysates with the food‐grade proteolytic activity (Alcalase 2.4 L, Flavourzyme 500 L, and Corolase PP). The result confirmed that the oxygen radical absorbance capacity (ORAC) and ferric reducing antioxidant power (FRAP) values of these hydrolysates ranged from 45.17 to 467.54 and from 1.06 to 21.59 μmol trolox equivalents/g protein, respectively. Four hydrolysates of trypsin, alcalase, and a combination of 2 sequential hydrolysates of first cold water protein extract (PF) derived from Porphyra columbina were analyzed (Cian and others 2012b). These hydrolysates exhibited antioxidant capacity of DPPH, TEAC, ORAC, and copper‐chelating activity. It has been also described that radical scavenging activities (TEAC and DPPH inhibition) were attributed by the residual cake protein hydrolysate of phycocolloids‐obtaining process of red seaweed P. columbina (Cian and others 2013). Identified peptides and protein hydrolysates from seaweed having potential antioxidant properties are summarized in Table 2. The transport path of peptides in the body is mainly affected by molecular size and structural properties, such as hydrophobicity. It has been described that peptides with 2 to 6 amino acids are absorbed more readily than proteins and free amino acids. Some other researchers reported that small (di‐and tripeptides) and large peptides (10 to 50 amino acids) can exhibit their biological functions at the tissue level, since they can cross intestinal barrier intact. However, in general, the chance of peptides to pass the intestinal barrier decreases as their molecular weight increases (Sarmadi and Ismail 2010). It has been recorded that presence of proline and hydroxyl proline results in peptide resistance to digestive enzymes, especially tripeptides with Pro‐Pro at the C‐terminal that are resistant to proline‐specific peptides (FitzGerald and Meisel 2000). Generally, most of the reported seaweed protein–derived peptides with antioxidant activity were those with low molecular weights (Wang and others 2010a, 2010b; Cian and others 2012a, 2012b, 2013, 2015).