Mycolic acids of Mycobacterium aurum: structure and biogenetic implications

Mycobacterium aurum (type strain) was analyzed for its mycolate content. Three types of mycolates were identified: di-unsaturated, oxo and dicarboxy mycolates, each type being constituted by two subtypes. The acid released by pyrolysis was identified as docosanoic acid. By use of mass spectrometry and oxidation techniques, the structures of these six subtypes of mycolates were elucidated. They contain Z and E double bonds, the latter having an adjacent methyl branch. No cyclopropane ring was observed. All the methyl branches occurring in these mycolates derived from methionine, the methyl-branched chiral center adjacent to the double bond having an R configuration. The structures of the major di-unsaturated mycolates suggest that they cannot be the precursors of oxo mycolates. The amount of long-chain secondary alcohol (2-octadecanol) obtained from the whole cells was found to be much greater than that expected from hydrolysis of wax ester mycolate (ester of 2-octadecanol and dicarboxy-mycolic acid). Further investigations showed that 2-octadecanol was also present in triacylglycerols, esterifying the omega-carboxyl group of long-chain fatty acids structurally related to dicarboxy-mycolates.