The utilization of sterols by insects

Abstract

The experimental data discussed in this review are fragmentary and frequently fail, in a most tantalizing fashion, to allow of precise interpretation. For reasons that are indicated, a shadow of uncertainty hangs over several of the nutritional data discussed. The problems inherent in working with the small amounts of material available from insects' tissues have also contributed to the difficulties of interpretation, and even when an unambiguous experimental result is obtained, it may well be hazardous to generalize from one or two species to all members of the class, which, after all, is the largest in the animal kingdom.Nevertheless, it is becoming clear that the sterols play a role in insect physiology that seems remarkably similar to that of cholesterol in the mammal. It is evident that both “structural” and metabolic functions are fulfilled, and there is evidence for the existence of mechanisms for the excretion of sterols and their metabolites. If the latter are excretory forms of physiologically active substances, it seems clear that some metabolites other than ecdysone must be involved, since polar derivatives of cholesterol have been detected in the tissues of adult insects in which, presumably, the prothoracic glands have degenerated and ecdysone is no longer secreted.Studies of the distribution and dynamic state of the tissue sterols of insects show that the sterols per se have diverse roles within the tissues, which for convenience may be described as “structural.” Many species of insects can modify the sterols of their diet and presumably such modifications serve to provide a structure that is more appropriate to one or all of the insect's requirements. It is not entirely clear whether phytosterols must always be converted to some extent to cholesterol, but one must conclude from the data available that species differ widely in the extent to which they carry out such conversions. This suggests that while there is probably a basic unity among all species with respect to the types of multimolecular structures into which the sterols are incorporated, there are species differences in the detailed structure of the immediate environment of the sterol molecule. A rewarding aspect of future research in this field may well be the analysis of the relationship between sterols of different structures and the other lipid molecules with which they are associated in the cells of different species. Such studies may shed new light on the manner in which sterols contribute to the stability of subcellular structures.It is almost certain that with the structure of ecdysone finally established as that of a steroid and its biogenesis from cholesterol demonstrated, there will be increasing interest in the functions of sterols and steroids in insects. It is hoped that this review shows that there is an emergent body of coherent biochemical and physiological information to serve as a basis for future work.