Effect of myofibrillar muscle proteins on the in vitro bioavailability of non-haem iron

It is well established that the bioavailability of non-haem iron from foods is enhanced by the presence of meat. However, the nature of the promoter in meat has not yet been characterised. The present study was designed to compare the effects of the myofibrillar protein fractions on the bioavailability of non-haem iron in an attempt to identify the 'meat factor'. Rabbit skeletal muscle was fractionated and whole muscle, myofibrillar protein, myosin and actin were isolated. Myosin was subjected to selective proteolysis with chymotrypsin and the heavy meromyosin, light meromyosin, rod region and head region were prepared. Protein fractions (1 g) were incorporated into 100 g semi-synthetic liquid meal and the in vitro dialysability of iron was determined. Egg albumin was used as a reference protein. When compared with egg albumin, all protein fractions significantly enhanced iron dialysability, except for light meromyosin which was inhibitory. Myosin had a greater enhancing effect than actin and, within myosin, the enhancing effect was greatest for the heavy meromyosin fraction. The enhancement appeared to coincide with the known distribution of cysteine residues in the myofibrillar proteins. The presence of the sulphydryl blocking agent, N-ethylmaleimide (NEM), in meals containing myosin reduced iron dialysability in a dose-related manner, but NEM had only a small effect in meals containing actin. Meanwhile, incorporation of cysteine into meals containing actin increased iron dialysability. The present results suggest that the enhancement of non-haem iron dialysability by meat is associated with myosin, in particular, with the heavy meromyosin region. Peptide fractions rich in cysteine residues, probably constitutes the 'meat factor'.