Carbamoyltransferase Reactions in Plants. A Survey for Enzymic Diversity and the Potential for Herbicidal Activity of Transition State Analogue Inhibitors

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Abstract
The Mr values for aspartate carbamoyltransferase (ACTase) from nine species were determined by gel filtration on Sephacryl S-200. The mean value was 104 000 and there was no evidence for differing size classes of plant ACTase. At 200 mmol m−3, the only nucleotide which caused any inhibition was UMP; there was no evidence for differing patterns of nucleotide regulation. Ornithine carbamoyltransferase (OCTase) was isolated from eight species and the Km values for ornithine and carbamoyl phosphate were determined. The Km values for ornithine ranged from 0.3 mol m−3 to 3.0 mol m-3; the Km values for carbamoyl phosphate ranged from 0.11 mol m−3 to 0.55 mol m−3. The Mr values for the various OCTase isolates were determined by gel filtration on Sephacryl S-200 and ranged from 148 000 to 167 000 with a mean of 158 000. When pure wild oat OCTase was subjected to SDS—PAGE analysis, a single protein of Mr 38 000 was detected. These values are consistent with a tetrameric structure for OCTase. We have demonstrated N-(phosphonacetyl)-L-aspartate (PALA) and N-(phosphonacetyl)-L-ornithine (PALO) to be very potent in vitro inhibitors of plant ACTase and OCTase, respectively. Of the seven species tested, the K1 values for PALA ranged from 30 μmol m−3 to 100 μmol m−3 and those for PALO ranged from 170 umol m−3 to 350 μmol m−3; the inhibition was competitive with carbamoyl phosphate as the variable substrate. Although PALA and PALO were very potent in vitro inhibitors, they were poor herbicides. Mechanisms are discussed which may account for the lack of herbicidal activity of PALA and PALO.