THE IN VITRO METABOLISM OF THIOPENTAL BY A FORTIFIED, CELL-FREE TISSUE PREPARATION OF THE RAT

¹ Department of Pharmacology, University of Michigan Medical School, Ann Arbor, Michigan

A fortified, cell-free, homogenate of rat liver capable of metabolizing thiopental has been described. The influence of pH, tissue concentration and thiopental concentration on the activity of this preparation has been investigated. It has been shown that for maximum activity cytochrome c, ATP, nicotinamide and a suitable substrate are necessary. Of those substrates employed, Krebs cycle intermediates (citrate, -ketoglutarate, succinate, fumarate, malate) function best. The overall reaction is aerobic and is fairly labile since activity is almost completely destroyed by incubation for a period of three hours at 38°C. Most of the activity is present in the large particulate material which separates out with low speed centrifugation. Homogenized tissues other than liver are capable of metabolizing thiopental when properly fortified. However, of these, liver is most active followed by kidney, brain, skeletal muscle, cardiac muscle, and jejunum, in that order.

The effects of a number of metabolic inhibitors on the metabolism of thiopental by this in vitro system have been determined. Cyanide (2 x 10^-2 M) inhibits oxygen consumption and thiopental metabolism completely. Antimycin A (3.6 x 10^-7 M), a succinoxidase inhibitor, and malonate (5 x 10^-2 M) both inhibit oxygen consumption approximately 70 per cent, the former suppressing thiopental metabolism almost completely, the latter having no significant effect. The possible explanations for these observations are discussed. 2,4-Dinitrophenol (1 x 10^-4 M) completely suppresses creatine phosphorylation by this system, inhibiting thiopental metabolism only 30 per cent. Carinamide (2 x 10^-4 M) produces no inhibition of metabolism of the thiobarbiturate.