Info

'From Bursell (1967), after various authors.

'The quantity of nitrogen excreted in the different products is expressed as a proportion of the nitrogen in the predominant end product.

FIGURE 18.3. Metabolic interrelationships of nitrogenous wastes. [After E. Bursell. 1967. The excretion of nitrogen in insects. Adv. Insect Physiol. 4:33-67. By permission of Academic Press Ltd. and the author.]

In addition to the enzymes for uric acid synthesis there are also uricolytic enzymes that catalyze degradation of this molecule in many insects (Figure 18.3). Uricase has a wide distribution within the Insecta. Active preparations of allantoinase have been obtained from many species, but the distribution of this enzyme appears to be rather restricted compared with uricase. Although there are reports that indicate the occurrence of allantoicase and urease in tissue extracts from a few insects, their presence should not be regarded as having been established unequivocally. In other words, when urea and ammonia are produced in significant amounts, they are probably derived in a manner other than by the degradation of uric acid. The existence of an ornithine cycle for urea production, such as is found in vertebrates, has not been proved conclusively, even though the constituent molecules of the cycle (arginine, ornithine, and citrulline) and the enzyme arginase have been identified in several species (Cochran, 1975). Cochran (1985) suggested that urea is merely a by-product of the biochemical conversion of arginine to proline, used in flight metabolism (Chapter 14, Section 3.3.5). Similarly, the way in which ammonia is produced (especially in those insects in which it is a major excretory molecule) is poorly understood. It is generally assumed to result from deamination of amino acids, but the precise way in which this occurs remains unclear.

It has been suggested that the most primitive state was that in which the complete series of uricolytic enzymes was present, and ammonia was the excretory material. As insects became more independent of water, selection pressures led to loss of the terminal enzymes and production of more appropriate excretory molecules. This simple view should be regarded with caution. Thus, in some caterpillars, diet can affect the nature of the nitrogenous waste. In certain insects substantial quantities of a particular nitrogenous waste molecule are produced, yet the appropriate enzyme in the uricolytic pathway has not been demonstrated, and vice versa; that is, the effects of other metabolic pathways may override the uricolytic system. In many insects (especially endopterygotes) the predominant nitrogenous excretory product changes during development. For example, in the mosquito Aedes aegypti urea is the principal nitrogenous waste in the (aquatic) larvae, while uric acid becomes dominant in pupae and adult females (von Dungern and Briegel, 2001). In Pieris brassicae (Lepidoptera) the major excretory product in the pupa and adult is uric acid; in the larva this compound constitutes only about 20% of the nitrogenous waste, allantoic acid being the predominant end product (Table 18.1). Indeed, in some Lepidoptera, the ratio of uric acid to allantoin may fluctuate widely from day to day (Razet, 1961, cited from Bursell, 1967). Of great interest will be determination of factors that stimulate inhibition or activation (degradation or synthesis?) of uricolytic enzymes so that the most suitable form of nitrogenous waste is produced under a given set of conditions.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

Get My Free Ebook


Post a comment