Isotopic acetate and isovalerate in the synthesis of ergosterol by Neurospora.

The utilization of deuteroacetate for the synthesis of ergosterol in an acetateless Neurospora mutant has been described in an earlier paper (1). In this work it was demonstrated that at least 25 per cent of the hydrogen atoms of ergosterol is derived directly from acetate. Since the use of deuterium as a tracer necessarily involves errors of unknown magnitude due to the lability of deuterium attached to carbon in certain organic compounds and loss of deuterium during condensations, it was thought desirable to use isotopic carbon as the method of tagging the sterol precursors; the incorporation of two possible precursors, acetate and isovalerate, was studied. Recent work (2) has indicated that isovalerate is very efficiently used for cholesterol synthesis in the rat. As in previous experiments, a mutant strain of Neurospora (Y-2492) has been used which is genetically incapable of forming 2-carbon units in the metabolism of sucrose, and hence must be provided with exogenous acetate for growth.’