Purification and comparative studies of several mitochondrial aspartate aminotransferases from avian liver

Regulation of 2-oxoglutarate metabolism in rat liver by NADP-isocitrate dehydrogenase and aspartate aminotransferase

Kinetic and regulatory properties of NADP-isocitrate dehydrogenase (NADP-IDH) and aspartate aminotransferase (AsAT) responsible for 2-oxoglutarate metabolism in the cytoplasm and mitochondria of rat liver were studied. Based on the subcellular location of these enzymes and their kinetic parameters (Km, Ksi) obtained with highly purified enzyme preparations, it is suggested that synthesis of 2-oxoglutarate should be mainly determined by cytoplasmic NADP-IDH (86% of the total activity in the cell), whereas its utilization should depend on cytoplasmic AsAT (78% of the total activity). AsAT from the rat liver was specified by substrate inhibition and also by changes in the enzyme affinity for the substrates under the influence of some intermediates of the tricarboxylic acid cycle: isocitrate, succinate, fumarate, and citrate. Key intermediates of nitrogen metabolism (glutamate, glutamine, and aspartate) are involved in the regulation of NADP-IDH and AsAT. These enzymes are regulated oppositely, and the catalytic activity of one enzyme can be stimulated concurrently with a decrease in the activity of the other. Obviously, carbon and nitrogen metabolism in the rat liver can be controlled through redistribution of 2-oxoglutarate between different metabolic processes via regulatory mechanisms influencing differently located forms of NADP-IDH and AsAT.

Kinetic studies of chloride inhibition in aspartate aminotransferase activity

The inhibitive effects of chloride anion on the activity of mitochondrial aspartate aminotransferase (L-aspartate: 2-oxoglutarate-aminotransferase EC. 2.6.1.1.) from chicken (Gallus domesticus) and turkey (Maleagris gallopavo) were studied. Steady-state velocities were obtained from a wide range of chloride concentrations. The data were fitted by rational functions of 0:2 and 1:2 for chloride, using a non-linear regression program which guaranteed the fit. The goodness of fit was improved by the use of a computer program that combined model discrimination, parameter refinement and sequential design. It was concluded that chloride aspartate aminotransferase inhibition requires a minimum velocity equation of 1:2 with regard to chloride, and a plausible kinetic mechanism for this experimental result was proposed.