Vitamin K dependent carboxylation of glutamate residues to .gamma.-carboxyglutamate in microsomes from spleen and testes: comparison with liver, lung, and kidney

Abstract

Vitamin K-dependent carboxylation of glutamate residues to .gamma.-carboxyglutamate was demonstrated in proteins of [rat] spleen and testes microsomes. The rate of carboxylation in spleen microsomes was 0.9% and testes 3% of that in liver microsomes per mg of microsomal protein. For comparison the rates of carboxylation in lung and kidney microsomes were 17 and 8%, respectively, of the rate in liver microsomes. The high rate in liver microsomes may be due to a high carboxylase level as indicated by the high rate of peptide carboxylation in liver microsomes. Protein carboxylation in liver microsomes was linear for only 15 min while carboxylation in microsomes from extrahepatic tissue persisted much longer so that the total protein carboxylation in lung microsomes was 60%, kidney 18% testes 12%, and spleen 9% of that occurring in liver microsomes. Protein carboxylation was higher in microsomes from extrahepatic tissues of rats fed a vitamin K-deficient diet as compared to animals fed a vitamin K-sufficient diet. Protein carboxylation in microsomes from extrahepatic tissues was greatly stimulated by Mg ions and was dependent upon the addition of dithioerythritol. NADH could partially replace the dithiol in spleen, testes, and lung, but NADH-dependent carboxylation was relatively low in kidney and liver microsomes. Dithiol-dependent carboxylation was completely blocked by 10 .mu.M warfarin, but NADH-dependent carboxylation was only slightly inhibited by 100 .mu.M warfarin. Menaquinone-3 was much more active than vitamin K1 in driving carboxylation. Solubilized microsomes catalyzed the carboxylation of glutamate residues to .gamma.-carboxyglutamate in a pentapeptide Phe-Leu-Glu-Glu-Leu. The rate of carboxylation in lung microsomes was 22%, testes 3.3%, kidney 1.9% and spleen 1.6% of the rate in liver microsomes.