METABOLIC ORIGIN OF STEROLS IN RUMEN PROTOZOA

Abstract

Metabolic origin of sterols found in rumen protozoa was investigated by the use of ¹⁴C-labeled precursors. Hydrogenation of unsaturated sterols by protozoa was shown by incubation with ¹⁴C-cholesterol and ¹⁴C-β-sitosterol, suggesting that stigmastanol and campestanol in protozoa could be derived from exogenous C₂₉ and C₂₈ sterols, respectively. Low redox potential was found to be required for this hydrogenation reaction. No evidence was obtained for the assumption that protozoal cholestanol is formed by dealkylation of C(24)-alkyl-sterols. Incorporation of the label from ¹⁴C-acetate and ¹⁴C-mevalonate into the sterol fraction of protozoa was shown. Therefore, it appears highly probable that cholestanol is formed by de novo synthesis. The radioactivity in the protozoal sterol fraction, however, was not high enough to conclude with definite certainty that rumen protozoa possess a sterol-synthesizing capacity; the possibility that contaminating organisms such as yeasts and fungi were responsible for the reaction could not be denied. Attempts to prove cholestanol synthesis by radio gas-liquid chromatography were unsuccessful due to low specific radioactivity of the protozoal sterols labeled. Thus, poor or no capacity of rumen protozoa to synthesize sterols may, at least in part, explain the sterol requirement of these protozoa. In addition, other conversion and degradation reactions of sterols are described.