Inhibition of the oxidation of glutamate and isocitrate in liver mitochondria at a specific NADP+-reducing site.

The cation-complexing carboxylic-acid antibiotic X-537A, at concentrations far below that required for ionophorous activity, selectively inhibits the oxidation of glutamate and isocitrate by liver mitochondria in steady-state 3. The site of inhibition has been localized specifically at the reduction of NADP(+). Glutamate and isocitrate dehydrogenases, the oxidation of NAD(+)-dependent substrates, pyridine nucleotide transhydrogenations, and the respiratory chain between NADH (or NADPH) and O(2) are unaffected by the antibiotic X-537A. Kinetic evidence, i.e., competition between chlorotetracycline (a fluorescence probe for membrane-bound bivalent cation) and X-537A, indicates that the NADP(+)-reducing, antibiotic-sensitive site is most probably associated with the inner mitochondrial membrane.