Iron-binding properties, amino acid composition, and structure of muscle tissue peptides from in vitro digestion of different meat sources

ABSTRACT:  Background: The enhancing effect of meat on nonheme iron bioavailability in humans is thought to be due to the release of low-molecular-weight (LMW) iron-binding peptides during digestion. Objective: To better characterize the LMW iron-binding peptides from meat digests. Methods: Cooked beef, chicken, cod, lamb, and pork myofibrillar or sarcoplasmic protein extracts, casein, and egg albumin were digested in vitro with pepsin or pepsin/pancreatin. Ultrafiltrates were analyzed for N and iron and further characterized by gel filtration with added ⁵⁹Fe, amino acid analysis, and LC-MS. Results: 84% to 98% of total iron in enzymic digests was associated with soluble LMW peptides (< 10 kDa) of the myofibrillar proteins compared to only 2% to 20% in the corresponding sarcoplasmic protein digests. Pepsin digestion alone of the myobrillar proteins generated > 80% soluble LMW iron, compared to < 5% with casein and egg albumin. Iron-binding peptides from myofibrillar protein with an estimated 2 kDa molecular mass were separated by gel filtration. Peptides in this fraction were enriched in aspartic and glutamic acid residues and included potential peptide fragments of myosin. Conclusion: LMW (< 10 kDa) peptides in enzyme digests of myofibrillar proteins were the major facilitators of iron solubility. Unlike with casein, egg albumin, and most sarcoplasmic proteins, these LMW peptides were generated on pepsin digestion. One group of iron-binding peptides had a mass of approximately 2 kDa and was enriched in glutamic and aspartic acids. Such early generation of a multitude of LMW iron-binding peptides could explain the enhancing effect of muscle tissue on iron absorption.