Isoenzymic studies of esterase, L-aspartate: 2-oxoglutarate aminotransferase, and carbohydrase, in Zea mays

Macdonald, Timothy
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Isoenzymes have been the object of intense studies in both plants and animals for the past ten years. Two recent and comprehensive reviews have been written on animal Isoenzymes (Latner, 1967) and plant isoenzymes (Shannon, 1968), and isoenzymes have been the object of a recent book (Wilkinson, 1966). The major technique used in the study of isoenzymes has been gel electrophoresis j a technique that was first described by Smithies (1955) and which has undergone comparatively little change since its first descriptions. The term isozyme was first introduced by Markert and Møller (1959) to describe. the different molecular forms of proteins which exhibit the same enzymatic specificity. In 1964, the term isoenzyme was adopted by the Standing Committee on Enzymes of the International Union of Biochemistry (Webb, 1964) as representing the multiple forms of an enzyme. The present concept of the term isoenzyme is as yet still rather broad in that precise criteria which must be fulfilled for two enzymes to be considered isoenzymes have not been established. At present then, the term isoenzyme refers to the existence of two or more forms of an enzyme within a single species. Gel electrophoresis has become an important tool in defining the action of the gene. Prior to the development of this technique, mutations resulting in differential gene function and the subsequent alterations in enzyme production were limited to impairment of enzymatic activities. Mutations, therefore, that altered but did not inactivate the enzyme went undetected. Schwartz (1960, 1962a, b, 1964a,b,c,d,e, 1965, 1967) has described seven alleles for the pH 7.5 esterases in maize. The different alleles specify esterase isoenzymes having different migration rates in starch gels. If an analysis on the basis of esterase activity had been the approach used in the study of the maize pH 7.5 esterases these different allelic forms would not have been detected. Other examples similar to this have been described (Latner, 1967; Shannon, 1968) . Regulation of the primary products of the gene (enzymes) has largely been confined to work with microorganisms (Jacob and Monod, 1961; Cline and Bock, 1966; Vogel and Vogel, 1967). Repression and induction models which involve regulator and operator genes have come from such studies. The concept of the operon containing a discreet operator cistron has been postulated. In higher organisms, on the other hand, there is an obvious lack of data with respect to the operon theory. Schwartz (1962) has postulated the existence of 'prime' alleles and 'standard' alleles for the pH 7.5 esterases in maize. The 'prime' alleles of this sys- tem are subject to regulation in the form of cessation of enzyme production at certain stages of development, while the 'standard' alleles are not subject to this regulation. The system of 'prime' and 'standard' alleles is analogous in some respects to the operon theory. Criteria for specifying a discreet operator cistron in this system have not been met largely because linkage between the regulatory portion of the 'prime' allele and the functional or structural portion of this locus has not been broken. The present study grew out of a classification of esterase polymorphism in the species Zea mays. Three hundred to four hundred inbreds, varieties, and races of maize were examined for esterase activity. A general description of the anodal esterase isoenzymes occurring in maize tissues is presented. The maize esterase isoenzymes were also studied on the basis of substrate specificity, inhibition and activation, changes occurring in. the isoenzymic pattern during development, and genetic analysis. A similar, but limited, study was made for maize transaminase IS0enzymes and several carbohydrase isoenzymes. It will be shown that the maize esterases constitute a heterogeneous and complex mixture of isoenzymes which show a certain amount of tissue specificity, variations in substrate utilization, variations in responses towards inhibitors and activators, and changes in the spectrum of isoenzymes during germination. In addition, nine new genetic loci are described for the esterase isoenzymes. Simple and complex genetic mechanisms are noted, and in one instance .regulation of enzyme production resulting in a model somewhat analogous to a 'prime' and 'standard' allele was found. A hybrid enzyme was found to be produced in heterozygotes involving two variant transaminase isoenzymes.
Thesis (Ph. D.)--University of Hawaii, 1969.
Bibliography: leaves [168]-178.
ix, 178 p illus., tables
Enzymes, Corn
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