The Effect of Diet and of Methionine Loading on Activity of Enzymes in the Transulfuration Pathway in Sheep

In three experiments, activity of hepatic enzymes associated with metabolism of methionine through the transulfuration pathway were studied with respect to possible effects of diet and methionine infusion per abomasum. In experiment 1 no differences in methionine adenosyltransferase (MAT) or cystathionine lambda-lyase (CGL) were detected between lucerne and wheaten straw diets, or between effects of fasting for 48 h and 96 h after feeding lucrene chaff as opposed to fasting after feeding wheaten straw. Fasting for 96 h resulted in a trend toward increasing CGL and MAT specific activities on both diets. In experiment 2 MAT was depressed significantly by infusion of methionine at 1.4 g/day and to a greater extent by infusion at 4.2 g/day, whilst CGL was not significantly affected. In experiment 3 MAT specific activity decreased significantly in response to both levels of methionine supplementation. Betaine-homocysteine methyltransferase activity was increased by methionine infusion. CGL decreased in all treatments but there was a larger decrease in those animals receiving methionine infusion. No significant changes were observed in relation to other enzymes examined which included cystathionine beta-synthase and threonine dehydratase. These observations are consistent with the hypothesis that in sheep the increase in methionine in blood plasma which occurs when methionine is absorbed in increased amounts may be due to reduced entry into the transulfuration pathway because of a repression of MAT activity.