Comparison of molecular properties of rat plasma and erythrocyte selenium-dependent glutathione peroxidase.

The molecular structure of plasma and erythrocyte selenium-dependent glutathione peroxidase (GSH-Px) was studied in rats drinking water containing [75Se]selenious acid, 1.3 mg Se/L. Substantial differences were found using three-step fractionation, including gel filtration of crude plasma and erythrocyte lysate, gel filtration of 75Se-GSH-Px treated by mercaptoethanol, and SDS-electrophoresis. Native plasma 75Se-GSH-Px, which exhibited a molecular weight (M(r)) of approx. 700,000, could be destroyed by mercaptoethanol action, resulting in disintegration of enzyme into several different 75Se-protein fragments and release of part of low-mol-wt 75Se. Native erythrocyte 75Se-GSH-Px M(r) value was found to be 113,000; two 75Se-protein fragments arose after mercaptoethanol treatment without 75Se release from the enzyme. The 75Se-subunits of 22,500 and 21,900 were isolated from plasma and erythrocyte 75Se-GSH-Px, respectively. Another minor 75Se-GSH-Px was identified in erythrocyte lysate (M(r) 214,000, subunit 22,100), which was considered to be a dimer of the above-mentioned erythrocyte enzyme. It can be assumed, based on these data, that native plasma GSH-Px, in contrast to erythrocyte enzyme, represents a high-molecular wt complex composed of several tetramers linked with S-S bonds. A certain part of selenium present in this complex is probably not selenocysteine and may be released with the mercaptoethanol treatment.