Propionate Metabolism by Bovine Liver Homogenates with Particular Reference to Stress of Lactation

Abstract

Nuclear-free liver homogenates were employed to study the rate of incorporation of specifically labeled propionate into methyl-malonate and tricarboxylic acid cycle intermediates. The liver tissue was homogenized in a hypotonic solution. The medium was supplemented with essential coenzymes and incubated for 15min at 37C. Reaction products were isolated by column chromatography. Liver samples were obtained by biopsy from dairy cows representing various stages of lactation and levels of milk production. Considerable variation was found among cows in their ability to metabolize propionate. The incorporation of 14C-label from propionate into metabolic intermediates beyond methylmalonate was positively correlated with liver vitamin B12 concentration. This assay criterion was a reflection of methylmalonyl-CoA mutase activity; however, to what extent variations in propionyl-CoA synthetase and carboxylase and racemase enzyme activities affected the magnitude of this parameter could not be assessed. The absence of stimulation by in vitro addition of coenzyme B12 was interpreted to suggest that the apoenzyme was saturated with coenzyme. A portion of the variation in ability to metabolize propionate could be accounted for by differences in certain lactation characteristics. In general, as the degree of stress of lactation increased, the ability to metabolize propionate decreased.