Phosphotransacetylase as a key factor in biological production of polyhydroxybutyrate

Phosphotransacetylase (Pta) catalyzes the reversible conversion of acetyl-coenzyme A (CoA) to acetyl phosphate. Polyhydroxybutyrate (PHB) synthase and accumulation were compared between a Pta-deficient mutant and the wild-type Escherichia coli, which were transformed with pAE100, coding for 3-ketothiolase, NADPH-dependent acetoacetyl-CoA reductase, and PHB synthase from Ralstonia eutropha. During the growth period, PHB synthase activity in the Pta-deficient mutant was lower than that in the wild type. PHB accumulation in the Pta-deficient mutant, however, was higher than that in wild-type cells grown in Luria-Bertani (LB) medium containing 1% glucose (high C:N ratio). The Pta-deficient mutant showed PHB accumulation even in LB medium (low C:N ratio), whereas wild-type cells showed no PHB accumulation. These data suggest the activation of PHB synthase by acetyl phosphate that is synthesized by Pta. A decrease in Pta activity probably causes some increase in acetyl-CoA as substrate for the PHB synthesis pathway, resulting in increased PHB accumulation.

Automated chemoenzymatic synthesis of no-carrier-added [carbonyl-11C]propionyl L-carnitine for pharmacokinetic studies

Propionyl-L-carnitine (PLC) is under development as a therapeutic for the treatment of peripheral artery disease, coronary heart disease and chronic heart failure. Three methods were examined for labelling PLC in its propionyl group with positron-emitting carbon-11 (t12 = 20.3 min), one chemical and two chemoenzymatic. The former was based on the preparation of [11C]propionyl chloride as labelling agent via 11C-carboxylation of ethylmagnesium bromide with cyclotron-produced [11C]carbon dioxide and subsequent chlorination. Reaction of carrier-added [11C]propionyl chloride with L-carnitine in trifluoroacetic acid gave [11C]PLC in 12% radiochemical yield (decay-corrected) from cyclotron-produced [11C]carbon dioxide. However, the radiosynthesis was unsuccessful at the no-carrier-added (NCA) level of specific radioactivity. [11C]Propionate, as a radioactive precursor for chemoenzymatic routes, was prepared via carboxylation of ethylmagnesium bromide with [11C]carbon dioxide and hydrolysis. NCA [11C]PLC was prepared in 68 min in 14% radiochemical yield (decay-corrected) from [11C]propionate via sequential conversions catalysed by acetate kinase, phosphotransacetylase and carnitine acetyltransferase. A superior chemoenzymatic synthesis of NCA [11C]PLC was developed, based on the use of a novel supported Grignard reagent for the synthesis of [11C]propionate and conversions by S-acetyl-CoA synthetase and carnitine acetyltransferase. This gave an overall radiochemical yield of 30-48% (decay-corrected). This synthesis was automated for radiation safety and provides pure NCA [11C]PLC in high radioactivities ready for intravenous administration within 25 min from radionuclide production. The [11C]PLC is suitable for pharmacokinetic studies in human subjects with PET and the elucidation of the fate of the propionyl group of PLC in vivo.

FAILURE OF ARSENATE TO UNCOUPLE THE PHOSPHOTRANSACETYLASE SYSTEM IN CLOSTRIDIUM ACIDIURICI

Sagers, Richard D. (Brigham Young University, Provo, Utah), Moshe Benziman, and Sigrid M. Klein. Failure of arsenate to uncouple the phosphotransacetylase system in Clostridium acidiurici. J. Bacteriol. 86:978-984. 1963.-The conversion of pyruvate to acetyl phosphate by extracts of Clostridium acidiurici required coenzyme A (CoA), an electron-carrier system (ferredoxin and nicotinamide adenine dinucleotide), and a divalent metal. Other cofactors may be involved but are not presently defined. The metal activates the system transferring acetyl units between CoA and phosphate. Acetyl CoA could be generated from pyruvate or from acetyl phosphate, but in both cases arsenate failed to uncouple the high-energy acyl compounds. The rate of acetyl transfer between acetyl phosphate and CoA was markedly decreased by 0.005 m arsenate, but the generation of acetyl CoA from pyruvate was essentially unaffected until the arsenate concentration exceeded 0.02 m. Close agreement was observed between the amount of pyruvate utilized and the amount of acetyl phosphate formed, both in the presence and absence of arsenate. The CoA-dependent exchange of P(32)O(4) ( identical with) with acetyl phosphate proceeded at a rate approximately one-eightieth of the rate of acetylation of CoA, indicating an equilibrium value for the phosphotransacetylase reaction similar to that observed for Clostridium kluyveri. The failure of arsenate to uncouple the C. acidiurici enzyme may indicate a high degree of specificity in relation to the acetyl unit acceptor, giving preference to phosphate over arsenate.

PURIFICATION AND PROPERTIES OF ENZYMES INVOLVED IN THE PROPIONIC ACID FERMENTATION

Allen, S. H. G. (Western Reserve University, Cleveland, Ohio), R. W. Kellermeyer, R. L. Stjernholm, and Harland G. Wood. Purification and properties of enzymes involved in the propionic acid fermentation. J. Bacteriol. 87:171-187. 1964.-Chromatographic procedures are described for the separation and purification of phosphotransacetylase, acetyl kinase, malic dehydrogenase and coenzyme A (CoA) transferase. Purity of the enzymes was judged by homogeneity in an ultracentrifuge and by specific activity. Phosphotransacetylase was obtained 85% pure with a specific activity of 27.1. The preparation of acetyl kinase was a homogeneous protein with a specific activity of 531. The malic dehydrogenase likewise was homogeneous with a specific activity of 938. The CoA transferase, which was about 56% pure with a specific activity of 42.6, is the purest preparation of this enzyme yet described. The pH optimum was 6.5 to 7.8, and the K(m) for succinyl-CoA in the transfer of CoA to acetate was found to be 1.3 x 10(-4)m; for acetate, in the same transfer, the K(m) was 7.0 x 10(-3)m; for succinyl-CoA to propionate it was 6.8 x 10(-5)m, and for propionate, in the same reaction, 6.2 x 10(-4)m. Methods are described for the enzymatic production of methyl-malonyl-CoA, malonyl-CoA, propionyl-CoA, acetyl-CoA, and succinyl-CoA. The role of these enzymes in the propionic acid fermentation as well as the possible mechanism responsible for the high yields of adenosine triphosphate from glucose are considered.

Elimination of CoASH interference from acetyl-CoA cycling assay by maleic anhydride

A method for the removal of CoASH from tissue extracts by maleic anhydride is described. It eliminates CoASH interference in the acetyl-CoA cycling assay using phosphotransacetylase and citrate synthase. Maleyl-CoA thioether does not hydrolyze under the conditions of the assay and allows a reduction in the number of blank samples during acetyl-CoA determination. The levels of acetyl-CoA in whole rat brain, isolated synaptosomes, and mitochondria were found to be 61, 8.6, and 31.3 pmol/mg of protein, respectively.

Measurement of choline acetyltransferase with [14C]acetate by a cycling procedure

A multiple enzyme and multisubstrate cycling system is described for the radiometric determination of cholineacetyltransferase (ChAT) activity in crude tissue homogenates. The methods employs [14C]acetate coupled with the enzymes acetate kinase (AK) and phosphotransacetylase (PTA) for the generation of [14C]acetyl CoA. By recycling it was possible to avoid product inhibition of ChAT by CoA, ATP was maintained constant by rephosphorylation of ADP. Kinetics of the individual enzyme reactions were studied and the parameters obtained were used to select appropriate conditions to maintain linearity of varying amounts ChAT activity over a sixty minute time course. The sensitivity of the method is limited only by the specific activity of commercially available isotope labeled acetate.

The content of coenzyme A, acetyl-CoA and long-chain acyl-CoA in human blood platelets

The amounts of coenzyme A (CoASH) and acetyl-CoA in the acid-soluble extract of human blood platelets were quantitated by an isocratic reversed-phase liquid-chromatographic system. The analytical column was Supelcosil LC-18, and the solvent was 220 mmol/l potassium phosphate, pH 4.0, and 120 ml/l methanol. Long-chain acyl-CoA was estimated by the released coenzyme A after an alkaline hydrolysis of the acid-insoluble material of the blood platelets. The median values in platelets from fourteen healthy persons were 30 pmol CoASH/mg platelet protein, 45 pmol acetyl-CoA/mg platelet protein, and 34 pmol long-chain acyl-CoA/mg platelet protein.