CONTROL OF GLUCOSE-UTILIZATION IN WORKING PERFUSED RAT-HEART

Metabolic control analyses of glucose utilization were performed for f our groups of working rat hearts perfused with Krebs-Henseleit buffer containing 10 mu M glucose only, or with the addition of 4 mM D-beta-h ydroxybutyrate/1 mM acetoacetate, 100 ana insulin (0.05 unit/ml), or b oth. Net glycogen breakdown occurred in the glucose group only and was converted to net glycogen synthesis in the presence of all additions. The flux of [2-H-3]glucose through P-glucoisomerase (EC 5.3.1.9) was reduced with ketones, elevated with insulin, and unchanged with the co mbination. Net glycolytic flux was reduced in the presence of ketones and the combination. The flux control coefficients were determined for the portion of the pathway involving glucose transport to the branche s of glycogen synthesis and glycolysis. Major control was divided betw een the glucose transporter and hexokinase (EC 2.7.1.1) in the glucose group. The distribution of the control was slightly shifted to hexoki nase with ketones, and control at the glucose transport step was aboli shed in the presence of insulin. Analysis of the pathway from 3-P-glyc erate to pyruvate determined that the major control was shared by enol ase (EC 4.2.1.1) and pyruvate kinase (EC 2.7.1.40) in the glucose grou p. Addition of ketones, insulin, or the combination shifted the contro l to P-glycerate mutase (EC 5.4.2.1) and pyruvate kinase. These result s illustrate that the control of the metabolic flux in glucose metabol ism of rat heart is not exerted by a single enzyme but variably distri buted among enzymes depending upon substrate availability, hormonal st imulation, or other changes of conditions.