Proline as an intermediate in the reductive deamination of ornithine to delta-aminovaleric acid

Abstract

Fresh extracts of cells of Clostridium botulinum reduced a limited amount of ornithine to delta-aminovaleric acid, but at high substrate concentrations a considerable amount of an amino compound accumulated which was neutral at pH 4.2. Aging of the extracts at -10 C or freezing and thawing resulted in the loss of the ability to produce delta-aminovaleric acid, but the ability to produce the neutral compound was retained. This compound was separated by column chromatography, and was found to be identical to dl-proline with respect to (i) R(F) upon paper chromatography, (ii) migration rates upon paper ionophoresis, (iii) spectrum of the product of the ninhydrin reaction, (iv) oxidation with d-amino acid oxidase, and (v) rate of reduction to delta-aminovaleric acid by cell extracts. The intermediate role of proline in the reduction of ornithine to delta-aminovaleric acid was indicated by (i) rate studies with and without an added electron donor and with and without inhibitors of proline reductase, (ii) the initial accumulation of radioactive proline to the exclusion of radioactive delta-aminovaleric acid from (14)C-l-ornithine in the presence of low levels of carrier proline, and (iii) the initial accumulation of proline at low levels prior to a significant accumulation of delta-aminovaleric acid in reaction mixtures in which the latter compound was the primary product after a longer incubation time. The conversion of ornithine to proline was the rate-limiting step in the presence of a good electron donor (alanine). The mechanism of the conversion of ornithine to proline has not been established. Preliminary data indicated that it may involve an oxidation to glutamic-gamma-semialdehyde and its equilibrium product, Delta(1)-pyrroline-5-carboxylic acid.