Purification and properties of rat brain pyruvate carboxylase

Inhibitors of Pyruvate Carboxylase

This review aims to discuss the varied types of inhibitors of biotin-dependent carboxylases, with an emphasis on the inhibitors of pyruvate carboxylase. Some of these inhibitors are physiologically relevant, in that they provide ways of regulating the cellular activities of the enzymes e.g. aspartate and prohibitin inhibition of pyruvate carboxylase. Most of the inhibitors that will be discussed have been used to probe various aspects of the structure and function of these enzymes. They target particular parts of the structure e.g. avidin - biotin, FTP - ATP binding site, oxamate - pyruvate binding site, phosphonoacetate - binding site of the putative carboxyphosphate intermediate.

Functional characterization and localization of acetyl-CoA hydrolase, Ach1p, in Saccharomyces cerevisiae

Acetyl-CoA hydrolase (Ach1p), catalyzing the hydrolysis of acetyl-CoA, is presumably involved in regulating intracellular acetyl-CoA or CoASH pools; however, its intracellular functions and distribution remain to be established. Using site-directed mutagenesis analysis, we demonstrated that the enzymatic activity of Ach1p is dependent upon its putative acetyl-CoA binding sites. The ach1 mutant causes a growth defect in acetate but not in other non-fermentable carbon sources, suggesting that Ach1p is not involved in mitochondrial biogenesis. Overexpression of Ach1p, but not constructs containing acetyl-CoA binding site mutations, in ach1-1 complemented the defect of acetate utilization. By subcellular fractionation, most of the Ach1p in yeast was distributed with mitochondria and little Ach1p in the cytoplasm. By immunofluorescence microscopy, we show that Ach1p and acetyl-CoA binding site-mutated constructs, but not its N-terminal deleted construct, are localized in mitochondria. Moreover, the onset of pseudohyphal development in homozygote ach1-1 diploids was abolished. We infer that Ach1p may be involved in a novel acetyl-CoA biogenesis and/or acetate utilization in mitochondria and thereby indirectly affect pseudohyphal development in yeast.

Acetyl-CoA hydrolase involved in acetate utilization in Saccharomyces cerevisiae

Acetyl-CoA hydrolase, catalyzing the hydrolysis of acetyl-CoA, is presumably involved in regulating the intracellular acetyl-CoA or CoASH pools. The yeast enzyme is encoded by ACHl (acetyl-CoA hydrolase) and the expression of ACH1 is repressed by glucose (Lee, F.-J.S., Lin, L.-W. and Smith, J.A. (1990) J. Biol. Chem. 265, 7413-7418). In order to study the biological function of the acetyl-CoA hydrolase, a null mutation (achl-1) was created by gene replacement. The mutation, while not lethal, slows down acetate utilization. In comparison to wild-type, homozygote achl-l diploids, the onset of sporulation was delayed. When measuring the levels of ACH1 mRNA and acetyl-CoA hydrolase activity, we demonstrated that ACHl was highly expressed during sporulation process. These results indicated that acetyl-CoA hydrolase in yeast cells involved in acetate utilization and subsequently affected the sporulation process.

Purification and characterization of an acetyl-CoA hydrolase from Saccharomyces cerevisiae

Acetyl-CoA hydrolase, which hydrolyzes acetyl-CoA to acetate and CoASH, was isolated from Saccharomyces cerevisiae and demonstrated by protein sequence analysis to be NH2-terminally blocked. The enzyme was purified 1080-fold to apparent homogeneity by successive purification steps using DEAE-Sepharose, gel filtration and hydroxylapatite. The molecular mass of the native yeast acetyl-CoA hydrolase was estimated to be 64 +/- 5 kDa by gel-filtration chromatography. SDS/PAGE analysis revealed that the denatured molecular mass was 65 +/- 2 kDa and together with that for the native enzyme indicates that yeast acetyl-CoA hydrolase was monomeric. The enzyme had a pH optimum near 8.0 and its pI was approximately 5.8. Several acyl-CoA derivatives of varying chain length were tested as substrates for yeast acetyl-CoA hydrolase. Although acetyl-CoA hydrolase was relatively specific for acetyl-CoA, longer acyl-chain CoAs were also hydrolyzed and were capable of functioning as inhibitors during the hydrolysis of acetyl-CoA. Among a series of divalent cations, Zn2+ was demonstrated to be the most potent inhibitor. The enzyme was inactivated by chemical modification with diethyl pyrocarbonate, a histidine-modifying reagent.

The physiological role of pyruvate carboxylation in hamster brown adipose tissue

1. Pyruvate carboxylase is present in brown adipose tissue mitochondria. 2. In isolated mitochondria, pyruvate, bicarbonate and ATP, the substrates for pyruvate carboxylase, are able to replace added malate in supplying a condensing partner for acetyl-CoA formed from beta-oxidation of fatty acids. 3. In brown adipocytes, pyruvate and CO2 increase the rate of norepinephrine-stimulated respiration synergistically. 4. The norepinephrine-stimulated respiration in brown adipocytes is diminished when pyruvate transport into the mitochondria is inhibited. 5. Pyruvate carboxylation increases the intramitochondrial level of citric acid cycle intermediates, as shown by titrations of malonate inhibition of respiration. 6. Pyruvate carboxylation can continuously supply the mitochondria with citric acid cycle intermediates, as evidenced by its ability to maintain respiration when oxoglutarate conversion to glutamate is stimulated. 7. Pyruvate carboxylation is necessary for maximal oxygen consumption even when drainage of the citric acid cycle for amino acid synthesis is eliminated. 8. Pyruvate carboxylation explains observed effects of CO2 on respiration in brown adipocytes, and may also explain the increased glucose uptake by brown adipose tissue during thermogenesis in vivo.

A plea for a comparative approach to evaluation of the physiological significance of regulatory mechanism observed in in vitro studies

Regulatory mechanisms proposed on the vasis of in vitro studies using purified enzymes should be evaluated for their physiological significance in different species and under various conditions. The procedures presently employed for such evaluation are examined, and it is suggested that difficulties of application or interpretation arise, especially in studies on highly differentiated organisms. The use of comparative enzymology as an alternative approach to this problem is proposed, using in the main data obtained in studies on pyruvate carboxylase to illustrate the potential of this method. Some problems which arise in the use of comparative studies in this context are considered.