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J B Hoek

Publications and source records attributed to J B Hoek.

104 records · Page 6Linked to original sources
PubMed1980

Kinetics and mechanisms of glutamate transport across the mitochondrial membrane.

The kinetics of glutamate influx and efflux on the glutamate-hydroxyl carrier have been measured and compared in rat liver mitochondria. At pH 7.4 and 25 degrees C, the Michaelis constants and V'max values were in agreement with the Haldane relationship when the alpha pH was accounted for. The Km values for glutamate influx and aspartate efflux on the glutamate-aspartate translocator are also reported. Extrapolation of the maximum velocities to 37 degrees and the intact liver provide values of 5.6 and 2.4 mmol/g dry wt/hr for glutamate influx and efflux, respectively, on the glutamate-aspartate translocator. Both translocators operate by a sequential mechanism with formation of a ternary complex. Their possible regulatory role in urea synthesis by liver is assessed.

Animals↗
PubMed1976

Nicotinamide-adenine dinucleotide-linked "malic" enzyme in flight muscle of the tse-tse fly (Glossina) and other insects.

1. A high activity of NAD-linked "malic" enzyme was found in homogenates of flight muscle of different species of tse-tse fly (Glossina). The activity was the same as, or higher than, that of malate dehydrogenase and more than 20-fold that of NADP-linked "malic" enzyme. A similar enzyme was found in the flight muscle of all other insects investigated, but at much lower activities. 2. ACa2+-stimulated oxaloacetate decarboxylase activity was present in all insect flight-muscle preparations investigated, in constant proportion to the NAD-linked "malic" enzyme. 3. A partial purification of the NAD-linked "malic" enzyme from Glossina was effected by DEAE-cellulose chromatography, which separated the enzyme from malate dehydrogenase and NADP-linked "malic" enzyme, but not from oxaloacetate decarboxylase. 4. The intracellular localization of the NAD-linked "malic" enzyme was predominantly mitochondrial; latency studies suggested a localization in the mitochondrial matrix space. 5. Studies on the partially purified enzyme demonstrated that it had a pH optimum between 7.6 and 7.9. It required Mg2+ or Mn2+ for activity; Ca2+ was not effective. The maximum rate was the same with either cation, but the concentration of Mn2+ required was 100 times less than that of Mg2+. Acitivity with NADP was only 1-3% of that with NAD, unless very high (greater than 10mM) concentrations of Mn2+ were present. 6. It is suggested that the NAD-linked "malic" enzyme functions in the proline-oxidation pathway predominant in tse-tse fly flight muscle.

Animals↗
PubMed1976

Evidence for a lipid dependence of mitochondrial nicotinamide nucleotide transhydrogenase.

1. The lipid dependence of mitochondrial nicotinamide nucleotide transhydrogenase from beef heart was investigated. With submitochondrial particles digestion of phospholipids by phospholipases A and C led to a partial inhibition that could not be readily reversed by phospholipids. 2. Extraction of neutral lipids including ubiquinone from lyophilized submitochondrial particles with pentane did inhibit the transhydrogenase, whereas further extraction with water/acetone led to a complete and apparently irreversible inhibition. 3. A partially purified preparation of transhydrogenase, depleted of lipids (and inactivated) by treatment with cholate and ammonium sulphate, was reactivated by various purified phospholipids but not by detergents or triacylglycerols. 4. It is concluded that mitochondrial transhydrogenase, catalyzing the nonenergy-linked transhydrogenase reaction, requires phospholipids specifically for its catalytic activity and not as dispersing agents. A mixture of phospholipids appears to fulfill this requirement better than the individual phospholipids.

Animals↗