Biochemical Genetics of Two Highly Polymorphic Esterases in 'Drosophila buzzatii'

Abstract

This thesis presents the results of a series of experiments which were undertaken in an attempt to describe some basic biochemical and physiological features of the two predominant adult esterase isozymes of the cactophilic species 'Drosophila buzzatii'. The two isozymes ESTERASE-1 and ESTERASE-2 were first characterised with respect to their ontogenic and tissue distributions. ESTERASE-1 was found to be located predominantly in the haemolymph, and although present in all stages of development showed highest titres in late third instar larvae and metamorphosis and a gain in young adults immediately after eclosion. In addition another esterase activity, EST-J was described during late larval and pupal development, and this enzyme appears to be related to the EST-1 isozyme. The EST-2 enzyme was found to be distributed primarily in the alimentary tract, and it also was found to be present in all life stages. However, EST-2 showed multiple peaks of activity corresponding approximately to the mid-late second and third larval instars, mid-pupal development and a broad peak of activity over the first week of adult life. Biochemical comparisons of the EST-1 and EST-2 isozymes revealed that these two enzymes differed for every character examined. ESTERASE-1 was apparently a dimer of molecular weight 128,000 daltons. It had a pH optimum in the vicinity of 8.0 to 9.0, was relatively stable at 50°C and showed a marked substrate preference for formate esters, and esters of propionic acid. By contrast, EST-2 was a monomer of molecular weight 54,000 daltons. It had a pH optimum around 7.0 to 7.5, was unstable at 50°C and showed a preference for esters of butyric acid. These two isozymes also differed in their response to inhibitors. These biochemical and physiological results permitted the establishment of fairly convincing homologies between the 'Est-1' and 'Est-2' loci of 'D. buzzatii', and the major 'β-' and 'α-Est' loci of other 'Drosophila' species in the 'virilis-repleta' radiation.