Growth hormone and liver mitochondria. Effects on cytochromes and some enzymes. |
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Authors: | V T Maddaiah C L Weston S Y Chen P J Collipp |
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Institution: | Division of Human Genetics, Departments of Medicine, Pharmacology, and Pediatrics, New York University Medical Center, 550 First Avenue, New York, New York 10016 USA |
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Abstract: | A reliable and reproducible assay was developed for measuring mitochondrial α-keto acid decarboxylase activity using ferricyanide as the electron acceptor. This method permitted the functional isolation and investigation of the decarboxylase step of the branched-chain α-keto acid dehydrogenases in rat liver mitochondria. Pyruvate and α-ketoglutarate decarboxylases are known to be separate and distinct enzymes from the branched-chain α-keto acid decarboxylases and were studied as controls. The relative specific activities of rat liver mitochondrial decarboxylases as measured by the ferricyanide assay showed that pyruvate and α-ketoglutarate were decarboxylated twice as rapidly as α-ketoisovalerate and four to ten times as fast as α-keto-β-methylvalerate and α-ketoisocaproate. The three branched-chain α-keto acids individually inhibit pyruvate and α-ketoglutarate decarboxylases. Inactivation of mitochondrial branched-chain α-keto acid decarboxylase activity by freezing and thawing and by prolonged storage resulted in a proportional decrease in decarboxylase activity toward each of the three branched-chain α-keto acids. However, hypophysectomy was found to increase decarboxylase activity with α-keto-β-methylvalerate to four times normal and with α-ketoisovalerate to three times normal, but the activity with α-ketoisocaproate was not changed. Hypophysectomy did not alter mitochondrial decarboxylase activity with pyruvate, α-ketoglutarate, or α-ketovalerate. The finding that hypophysectomy differentially alters the mitochondrial decarboxylase activity with the three branched-chain α-keto acids suggests either that there is more than one substrate-specific enzyme with branched-chain α-keto acid decarboxylase activity or that there is a modification of one enzyme such that the catalytic activity is selectively altered toward the three substrates. |
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