The strategies of energy conservation in helminths.

From the above discussion it is quite obvious that the bioenergetics in helminths are different in many ways from those found in higher organisms. All adult helminths appear to be able to consume oxygen when it is available but none of them can use it to drive the pathways of complete substrate degradation, like typical aerobic organisms, as a major strategy for energy generation. These properties hold also true for those worms residing in a highly aerobic environment, such as the blood stream or the muscle and lung tissues. Although in a number of recent studies oxygen was found to play apparently a greater role in the bioenergetics of adult helminths than originally thought, energy-generating mechanisms in adult worms seem to place greater emphasis on fermentations and anaerobic electron transport processes. These exhibit relatively low energy conservation efficiencies and result in the formation of a variety of organic end products, most of which must be excreted. The obvious correlation between the type of bioenergetic strategy operative in a particular helminth species and its environmental conditions is not well understood. The increased capacity to generate chemical energy and key metabolites of helminths possessing multiple fermentations and anaerobic respirations may give the organism greater versatility and metabolic flexibility to respond to the environmental changes observed in its corresponding habitat. Other helminths, such as schistosomes and filariids, which have continuous access to a fairly constant nutrient supply, were found to depend primarily on the more inefficiently functioning and primitive strategy of glycolysis for energy production. The reason for the occurrence in helminths of limited oxidative capacities is not completely clear. It may be assumed that the variety of alternative anaerobic pathways have evolved in response to the lack of a circulatory system and/or to the specific, often peculiar, environmental conditions prevailing in most parasitic habitats. An alternative idea put forward by Barrett [8] is that helminth metabolism represents a form of biochemical economy. Most endoparasites have an abundant supply of food and swim as if in a land of Cockain, obviously without any need to extract a maximum amount of chemical energy from the nutrients they take up. On the other hand, the fact that free-living and other larval or juvenile stages of helminths often have a typical aerobic bioenergetic pattern is a clear indication that the DNA of these organisms carries the genetic message for all the enzymes involved in complete substrate degradation.(ABSTRACT TRUNCATED AT 400 WORDS)