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Reactions of D-glyceraldehyde 3-phosphate dehydrogenase facilitated by oxidized nicotinamide-adenine dinucleotide.

ABSTRACT: Transient kinetic methods have been used to study the influence of NAD(+) on the rate of elementary processes of the reversible oxidative phosphorylation of d-glyceraldehyde 3-phosphate catalysed by d-glyceraldehyde 3-phosphate dehydrogenase. In the pH range 5-8 NAD(+) is bound to the enzyme during the following elementary processes of the mechanism: phosphorolysis of the acyl-enzyme, its formation from 1,3-diphosphoglycerate and the enzyme and the formation and breakdown of the glyceraldehyde 3-phosphate-enzyme complex. The rates of these four elementary processes only equal or exceed the turnover rate of the enzyme when NAD(+) is bound and are as much as 10(4) times the rates in the absence of NAD(+). Autocatalysis of the reductive dephosphorylation of 1,3-diphosphoglycerate occurs when glyceraldehyde 3-phosphate release is rate determining because NAD(+) is a reaction product. An important feature of the enzyme mechanism is that the negative-free-energy change of a chemical reaction, acyl-enzyme formation, is linked in a simple way to the positive-free-energy change of a dissociation reaction, NAD(+) release.

SUBMITTER: Trentham DR

PROVIDER: S-EPMC1176687 | biostudies-other | 1971 Mar

REPOSITORIES: biostudies-other

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Project description:1. The concentrations of NADP and NADPH(2) in homogenates of rat liver (expressed as mug./g. wet wt. of tissue homogenized) were compared with values obtained from intact samples of liver taken from the same female rat. With 0.25m-sucrose alone as the suspending medium, or in combination with tris buffer or 0.01-0.1m-nicotinamide, considerable decreases in the sum of the NADP+NADPH(2) concentrations were occasionally observed during 30min. storage of homogenates at 0 degrees . However, addition of 0.5m-nicotinamide+5mm-tris buffer to 0.25m-sucrose for use as a suspending medium maintained the sum of the NADP+NADPH(2) concentrations in homogenates at the level found in intact tissue for at least 30min. at 0 degrees . 2. The effects of freezing intact tissue and homogenates in liquid nitrogen before the extraction of NADP and NADPH(2) were studied. Freezing alone appears to convert a significant amount (approx. 30%) of liver NADPH(2) into an equivalent amount of NADP in intact tissue. This is discussed in terms of the ;bound NADP' reported by Burch, Lowry & Von Dippe (1963). 3. The intracellular distributions of NADP and NADPH(2) in intracellular fractions of rat liver were studied by using a modified centrifuging scheme that allows extraction of the isolated fractions to be performed within 45min. of killing the animal. Approx. 50% of the total NADP+NADPH(2) was found in the large-particle fractions and the remaining 50% was mostly in the soluble fraction of the cell. 4. Further investigations are reported on the nature of ;bound NADP' in rat liver. Most of this material appears associated with the ;nuclear' (containing nuclei, debris, erythrocytes etc.) or large-mitochondrial fractions, or both, obtained by low-speed centrifuging of rat-liver homogenates. 5. Although in some experiments the variations produced in the concentration of NADPH(2) present in large-particle fractions were followed by similar changes in that of ;bound NADP', in other cases no such direct relationship was obtained. Addition of phenazine methosulphate, for example, consistently lowered the concentration of NADPH(2) yet raised the concentration of ;bound NADP' in rat-liver mitochondrial fractions.

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Reactions of D-glyceraldehyde 3-phosphate dehydrogenase facilitated by oxidized nicotinamide-adenine dinucleotide.

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