Studies of Prothrombin Biosynthesis in Cell-free Systems: III. REGULATION BY VITAMIN K AND WARFARIN OF PROTHROMBIN BIOSYNTHESIS IN RAT LIVER MICROSOMES

Abstract

Abstract The de novo biosynthesis of prothrombin by rat liver microsomes has been studied under various conditions of vitamin K nutriture and 4-hydroxycoumarin drug treatment. Incorporation of label from l-[U-14C]leucine into prothrombin as measured by specific antibody and a radioimmunoassay of net prothrombin biosynthesis with 125I-labeled rat prothrombin were both employed. It was found that the rate of prothrombin biosynthesis in vitro correlated well with the plasma prothrombin levels observed in vivo under various conditions. Liver microsomes from vitamin K-deficient rats or warfarin-treated rats showed a negligible prothrombin synthesis when compared with normal control preparations. When deficient or anti-coagulated rats were treated with doses of vitamin K in vivo required to restore circulating prothrombin levels toward normal, a large increase in the incorporation of isotopic amino acid into prothrombin was observed in vitro which paralleled measurements of net synthesis of prothrombin by radioimmunoassay. Under these conditions, total protein synthesis and serum albumin synthesis as measured by specific antibody were unaffected. When vitamin K1 was added in vitro to microsomes from deficient rats, no effect was observed on the negligible prothrombin synthesis. Similarly, when warfarin was added to microsomes from normal rats, no diminution in prothrombin biosynthesis was observed. Since unfortified rat liver microsomes engage only in peptide chain elongation and not initiation, it was concluded from these data that vitamin K and warfarin probably regulate the initiation of prothrombin biosynthesis in rat liver cells.