Control of fructose and citrate synthesis in guinea pig seminal vesicle epithelium.

Abstract

Glucose utilization, biosynthesis of fructose and citrate, and certain aspects of energy metabolism were studied in a homogeneous preparation of mating guinea pig seminal vesicle epithelium. Under aerobic conditions, ATP:ADP ratios between 6 and 7 were maintained attesting to the viability and metabolic integrity of the preparation. There were multiple differences between seminal vesicle epithelium and smooth muscle on comparison of concentrations of 19 different metabolites including the adenine nucleotides. In seminal vesicle, glycolysis was rapid under anaerobic conditions (approximately 25 mumol times g-1 times hour-1 during the first 3 min) and also occurred under aerobic conditions (approimately 5 mumol times g-1 times hour-1). Anaerobically, the concentration of fructose diphosphate increased 2-fold and that of fructose 6-phosphate decreased to one-third of control values, consistent with regulation at the level of phosphofructokinase. ATP and total adenine nucleotides decreased rapidly and, by 3 min, had attained steady state values of about one-third and one-half of control values, respectively. Under aerobic conditions, the rate of fructogenesis increased with increasing concentrations of added glucose, reaching maximum (3 mumol times g-1 times hour-1) at 8 to 10 mM glucose and half-maximum at 2 mM glucose. Simultaneously, sorbitol synthesis occurred at rates that were similarly influenced by the concentrations of added glucose. The highest rate of fructogenesis (6.5 mumol times g-1 times hour-1) occurred during aerobic incubation immediately after a period of anaerobic incubation: exogenous substrate was not required. This could be prevented by addition of D-glyceraldehyde which was converted to glycerol stoichiometrically because of its preferential (versus glucose) reduction by aldose reductase. Our data are consistent with the sorbitol pathway of Hers as the major route of fructose biosynthesis. The rate of citrate syntehsis increased with increasing concentrations of added pyruvate. The maximum rate (3.4 mumol times g-1 times hour-1) was achieved with 2 mM pyruvate. Added glucose failed to support citrate synthesis to the same extent as did added pyruvate. The highest rate of citrate synthesis (8.0 mumol times g-1 times hour-1) occurred simultaneously with the highest rate of fructogenesis (after the anaerobic-aerobic transition). Exogenous substrate was not required.