The effect of molecular weight and amino acid composition on antioxidant properties of peptide components of pumpkin seed protein hydrolysate

The formation of free radicals in aerobic organs is inevitable. These radicals cause cells and tissues damages. Oxidation of lipids by producing harmful compounds and as a result spoilage of food products, is one of the most important concerns of the food industry. The use of synthetic antioxidants is tightly controlled due to their harmful effects on human health. Therefore, scientists' research has turned to identification and extraction of natural antioxidants compounds. Bioactive peptides are specific protein components that are inactive at the primary protein source and produced by enzymatic hydrolysis, microbial fermentation, and chemical synthesis. These peptides have different health-promoting properties such as antihypertensive, anti-cholesterol, anti-cancer, strengthening the immune system and antioxidant properties that are influenced by their amino acid composition and sequence. The aim of this study was to investigate the effect of molecular weight and amino acid composition on the antioxidant properties (DPPH radical scavenging activity, Fe reducing power and ABTS radical scavenging activity) of peptide components of pumpkin seed protein hydrolysate.

Pumpkin seed protein hydrolysate was prepared under optimum conditions based on the results of previous research and then the effect of molecular weight distribution on the bioactive properties of the resulting fractions and also the relationship between amino acid composition of these peptides and their antioxidant properties after separation and purification by size exclusion chromatography (SEC) and reverse phase high pressure liquid chromatography (RP-HPLC) was examined.

The results of size exclusion chromatography showed that fractions 49 to 56 and 49 to 63 with a molecular weight in range of 9000 to 13,000 daltons have highest DPPH and ABTA radical scavenging activity and after further purification of these fractions by RP-HPC, this ability in fractions with a holding time of 22 minutes showed the highest level. The results of amino acid analysis showed that fractions with high antioxidant capacity contain high amounts of hydrophobic amino acids (about 33.5%), which include non-polar aliphatic groups (such as valine, isoleucine, proline, leucine, tryptophan, and tyrosine). In other words, low molecular weight peptides, with aromatic and hydrophobic amino acids have higher antioxidant activity.

Pumpkin seed protein hydrolysate with peptide components with significant antioxidant properties, can compete with synthetic antioxidants used in the food industry, so it can be used as a natural preservative in food formulations to produce functional products with the aim of increasing public health level and also in medicinal purposes.