Effects of Diet, Pregnancy, and Lactation on Enzyme Activities and Gluconeogenesis in Ruminant Liver

Abstract

Changes of activities of liver enzymes and of metabolism during pregnancy and lactation have been documented in studies with rodents. Therefore, a study of activities of ruminant liver enzymes and metabolism was undertaken. Several changes of activities of enzymes involved in lipogenesis and gluconeogenesis were observed. However, magnitudes of enzyme change were less than expected from major changes in liver function that accompany lactation. Also, a number of enzymatic changes expected from rodent data were absent. Effects of breed and diet on enzyme activities in cow livers were minimal or absent. Changes in rates of glucose metabolism from lactation were greater than implied by enzyme adaptations. Analysis of kinetic data according to Michaelis-Menten kinetics indicated that apparent maximal velocities of conversion of lactate and propionate to glucose by liver slices from cattle ranged from 10 to 30 mumoles/(g.h) and apparent coefficients were 1.8 to 2.0- mM. Rates of gluconeogenesis with lactate plus propionate were not additive at saturating concentrations. The background data on enzymatic activities, glucose metabolism, lipogenesis, and gluconeogenesis in ruminant liver indicate that sophisticated studies with isolated hepatocytes from ruminants will be required to clarify metabolic adaptations, metabolic patterns, and regulatory mechanism in this tissue.