Comparative enzymology of wheat germ aspartate transcarbamoylase and related studies
Studies on aspartate transcarbamoylase from various organisms have been reviewed. The amino acid composition, isoelectric point and peptide maps of purified wheat germ aspartate transcarbamoylase have been elucidated and compared to the E.coli enzyme. Three types of essential amino acid residues have been demonstrated by chemical modification reagents. Essential arginine and lysine residues were studied by reaction with phenylglyoxal and pyridoxal 5'-phosphate respectively. In each case one or more residues were found at the active site, the reaction of which showing many similarities to previous studies on the E.coli catalytic subunit. One exceptionally reactive histidyl residue has been studied in wheat germ and E.coli aspartate transcarbamoylase. Reaction of these histidyl residues with diethylpyrocarbonate again indicated many similarities between the two enzymes. The position of the essential histidyl residue has been located in the published sequence of the E.coli enzyme and found to agree well with X-ray crystallographic and other evidence. The sequences adjacent to the essential histidyl and lysyl residues in the wheat enzyme were elucidated and compared to those of the E.coli enzyme. Attempts to induce the overproduction of aspartate transcarbamoylase in a carrot cell suspension culture by PALA were unsuccessful. These cells were instead found to have an innate mechanism for the detoxification of the drug. The nature of this detoxification mechanism has been partly elucidated.