Reversible activation and inactivation of mitochondrial malate dehydrogenase with para-hydroxymercuribenzoate

Modulation of heart muscle mitochondrial malate dehydrogenase activity. II. p-Mercuribenzoate activation, model of a possible allosteric control mechanism for substrate homeostasis

“…Gross subunit structure as determined by sucrose density gradient centrifugation was similar for native and PMB-activated enzyme as well as inactivated enzyme. This finding apparently rules out the suggestion of Devenyi et al (1966) that activation may be an association phenomenon, since no aggregate larger than native was discerned. The pH optimum of native and activated enzyme were also similar, while activated enzyme activity was more heat labile than native until about 80 % inactivation had occurred.…”

Modulation of heart muscle mitochondrial malate dehydrogenase activity. I. Activation and inhibition by p-mercuribenzoate

“…Optimum activation was observed with formation of four PMB-mercaptide bonds/mole of enzyme. DPNH-X was not found to inhibit the reaction of malate oxidation (Devenyi et al, 1966). It would thus seem more likely that reaction of PMB with enzyme sulfhydryls is responsible for the activation.…”

“…fructose diphosphatase by PMB and fluorodinitrobenzene (Cremona etal., 1965; Rosen and Rosen, 1966), and liver glutamate dehydrogenase by methyl mercuric chloride and PMB (Hellerman et a/., 1958; Bitensky et al, 1965; Silverstein and Sulebele, 1969b). An activation of pig heart M-MDH by PMB which was ascribed to reaction of the PMB with DPNH-X has independently been recently briefly reported (Devenyi et a!., 1966). The present communication describes the activation and inactivation process of heart mitochondrial malate dehydrogenase with PMB and other sulfhydryl reagents and some factors affecting it.…”

“…In this case, however, NADH was considerably more effective than hematoporphyrin. Devenyi et al (1966) have independently reported activation of native pig heart M-MDH by reaction with 2 moles of PMB/subunit (4 PMB/mole of enzyme) and inactivation of photooxidized M-MDH by reaction with 8 PMB/mole of enzyme at 0°. Activatability of native M-MDH was directly correlated with DPNH-X content (up to one/mole of enzyme) of various enzyme preparations and inversely proportional to mercaptide-bond formation (up to four/mole of enzyme) and inhibitability of photooxidized enzyme.…”

“…The failure of Devenyi et al (1966) to observe formation of more than four PMB-mercaptide bonds/mole of enzyme at 0°w ith native enzyme in the presence of excess PMB may be due to lack of sufficient time of observation since the sulfhydryls beyond the four apparently involved in activation may react more slowly; reaction of sulfhydryls beyond 8/mole clearly occurs more slowly. The observation by these workers of formation of eight PMB-mercaptide bonds/mole of photooxidized M-MDH with simultaneous inactivation is likely due to more rapid PMB-mercaptide formation than with native enzyme, which was therefore observable under the conditions used.…”