Enzyme from maize kernels

Dohlert, D. C., and Knutson, C. A. 1991. Two classes of starch debranching enzymes from developing maize kernels. J. Plant Physiol. 138, 566-572. 

Vos-Scheperkeuter, G. H., de Wit, J. G., Ponstein, A. S., Feenstra, W. J., and Witholt, B. 1989. Immunological comparison of the starch branching enzymes from potato tubers and maize kernels. Plant Physiol 90,75-84. 

Eukaryotic DNA Transposons McCllntock s original discovery of mobile elements came from observation of certain spontaneous mutations in maize that affect production of any of the several enzymes required to make anthocyanln, a purple pigment in maize kernels. Mutant kernels are white, and wild-type kernels are purple. One class of these mutations is revertlble at high frequency, whereas a second class of mutations does not revert unless they occur in the presence of the first class of mutations. McClintock called the agent responsible for the first class of mutations the activator (Ac) element and those responsible for the second class dissociation (Ds) elements because they also tended to be associated with chromosome breaks. 

The process of endosperm development clearly involves the synthesis of many proteins, not only those enzymes involved in general cell metabolism and the synthesis of reserves, but also those proteins which are themselves laid down as storage material. Biosynthetic activity of the maize kernel increases from about the 8th day after anthesis up to about the 28th day (Fig. 3.5), after which it declines. The initial increase may, in part, be a reflection of increased enzyme synthesis within individual cells of the maize endosperm and in part an increase in the number of cells commencing protein synthesis as they approach... 

The search for better cell-free preparations for studying amino acid incorporation and protein synthesis in vitro has continued, and a number of novel systems have been developed. Satake and co-workers 10) studied the incorporation of leucine into microsomal protein by a purified system from guinea pig brain, and found it to be in all respects similar to the classic microsome-pH 5 enzyme systems of rat liver and ascites tumor cells (cf. 9S, 53) (see also Table X). Other systems of this type have been obtained from the developing endosperm of maize kernels and from 2-day old maize seedlings by Mans and Rabson 11). No net synthesis of protein could be observed in either of these systems. 

The development of amylase activity in extracts of embryo-free and of GA3-treated, embryo-free maize kernels has been determined. The increase in amylase activity was accompanied by the appearance of several starch-degrading enzymes. Actinomycin D and cycloheximide prevented the amylolytic activity from developing. Other results indicated that the development of a-amylase activity in embryo-free maize kernels does not depend on gibberellic acid, but involves the de novo synthesis of protein. 

The next two reactions in the pathway are catalyzed by aspartate semialdehyde dehydrogenase and homoserine dehydrogenase, respectively (Fig. 2). Both enzymes were initially detected in extracts of pea seedlings (Sasa-oka and Inagaki, 1960 Sasaoka, 1961) and subsequently characterized as B stereospecific with respect to hydride transfer from the dihydropyridine ring of NADPH (Davies el al., 1972). Aspartate semialdehyde dehydrogenase has also been detected in extracts of maize shoots, roots, developing kernels,... 

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