Behavior of sterols of insect muscle during homogenization and differential centrifugation.

Labeled sterol introduced in vitro into a homogenate of insect muscle became distributed in the various subcellular fractions in a manner similar to that obtained by fractionation of muscle of insects to which labeled sterol had been administered in vivo. If two differently labeled sterols, cholesterol and cholestanol, were added to the in vitro preparation, the ratio of concentrations of the two compounds remained the same for the supernatant as for the particulate fractions. This was true for widely different total concentrations of the sterols. This result differs from that previously reported for the in vivo incorporation of these sterols and is viewed as further support for the proposal that some form of structural selectivity regulates the in vivo incorporation of the sterols into the membranous parts of the cell. Mitochondria into which labeled sterols had been incorporated under various conditions, both in vitro and in vivo, were combined with a second, unlabeled homogenate which was then fractionated in the usual way and assayed for radioactivity to assess the extent of redistribution of labeled sterol to other fractions. The results indicate that in the usual homogenization and fractionation procedure there is little redistribution of mitochondrial sterol that has been incorporated under long-term in vivo conditions. Sterol taken up by the mitochondria in vitro was redistributed to a significantly greater extent and somewhat similar results were obtained with sterol incorporated under short-term in vivo conditions. No differences between cholesterol and cholestanol with respect to their tendency to redistribute to other subcellular fractions could be detected in any of these experiments.