The stereoisomers of α∈-diaminopimelic acid. 3. Properties and distribution of diaminopimelic acid racemase, an enzyme causing interconversion of the ll and meso isomers

Abstract

The enzyme diaminopimelic acid racemase converts either LL- or meso-diamino-pimelic acid into a mixture of the 2 isomers. It has no action on DD-diaminopimelic acid. This enzyme, acting in sequence with diaminopimelic acid decarboxylase (which is specific for meso-diaminopimelic acid), is responsible for the apparent decarboxylation of LL-diamino-pimelic acid by many acetone-dried bacteria. The properties of diaminopimelic acid racemase were examined in cell-free extracts of Escherichia coli 26-26, which were free from decarboxylase. The pH optimum is about 7.7-8.3. The enzyme has a sensitive sulfhydryl group, but can be stabilized as an inactive Hg complex and subsequently reactivated. It is not activated by pyridoxal phosphate, but is inhibited by carbonyl-binding reagents; this inhibition is reversed by thiols. Diaminopimelic acid racemase is even more widely distributed among bacteria than is the decarboxylase. The distribution of these enzymes bears no relation to the occurrence of the isomers of diaminopimelic acid. Only among the Streptococcaceae were both these enzymes and diaminopimelic acid absent. Decarboxylase was absent from most Bacillaceae, but racemase was usually present.