[Structural and metabolic study of the mycolic acids of Mycobacterium fortuitum]

The biosynthesis of mycolic acids was studied in whole cells of Mycobacterium fortuitum. At first the structures of the main mycolates produced by the used strain were established as diunsaturated and epoxymycolates. By using [1-14C]acetate as a radiotracer of the lipid synthesis, it was observed that the turnover of the mycolates during the exponential phase of growth of M. fortuitum is fast enough to make very difficult the identification of their precursors. If the growth of the bacterial cells is stopped or highly diminished, by the removal of a large part of their nutritional medium, mycolate synthesis, in contrast to the synthesis of other fatty acids, is stopped as shown by incubation of the concentrated bacterial culture with [1-14C]acetate. After removal of aliquots of the sedimented bacteria at intervals, during several hours, mycolate synthesis resumes when the cell concentration becomes lighter. In these conditions the sequence of radiolabeling of mycolates and of their potential precursors (tetracosanoate and meromycolates) can be observed. In spite of their low accumulation, tetracosanoate and meromycolates were isolated and purified and their specific radioactivity, after different incubation times, could be measured. The results are in agreement with the hypothesis that meromycolates are condensed with tetracosanoate to produce mycolates. However, because of the large differences of isotopic dilution of these two precursors inside the mycolate molecule, this hypothesis, generally taken as evidence, has to be modified. A hypothetical pathway of the mycolate synthesis is proposed, taking into account all these observations.