Proline as an Intermediate in the Reductive Deamination of Ornithine to δ-Aminovaleric Acid

Abstract

Fresh extracts of cells of Clostridium botulinum reduced a limited amount of ornithine to δ-aminovaleric acid, but at high substrate concentrations a considerable amount of an amino compound accumulated which was neutral at p H 4.2. Aging of the extracts at −10 C or freezing and thawing resulted in the loss of the ability to produce δ-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 δ-aminovaleric acid by cell extracts. The intermediate role of proline in the reduction of ornithine to δ-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 δ-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 δ-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-γ-semialdehyde and its equilibrium product, Δ ¹ -pyrroline-5-carboxylic acid.