Putrescine carbamoyltransferase activity in pea is associated with the ornithine carbamoyltransferase polypeptide and is not involved in putrescine synthesis under physiological conditions
1993
Nichols, H.F. | Slocum, R.D.
Putrescine carbamoyltransferase (PutCT) has been postulated to function in the synthesis of putrescine (Put) from an N-carbamoylputrescine (NCPut) intermediate in plants. In pea, PutCT activity was associated entirely with ornithine carbamoyltransferase (OCT) protein, which was purified to homogeneity using an immobilized transition-state analog inhibitor ((delta)N-(phosphonacetyl)-L-ornithine). No evidence for a separate PutCT enzyme, similar to that in Streptococcus, or PutCT activity associated with a 'putrescine synthase'-type multifunctional enzyme was found. OCT carried out the carbamoylation of Put and other diamine and polyamine substrates inefficiently and at non-physiological pH (Put carbamoylation: pH 10.8 optimum, Vmax 0.11 microkatals/mg protein, Km = 6.7 mM for Put and 1.0 mM for carbamoyl-P), when compared with ornithine carbamoylation (pH 8.5 optimum, Vmax = 313.9 microkatals/mg protein, Km = 4.4 mM for ornithine and 0.6 mM for carbamoyl-P). Different subcellular compartmentation of PutCT activity (chloroplast) and the NCPut substrate (cytosol), coupled with a thermodynamically-unfavorable reverse reaction (i.e., Put synthesis from NCPut), suggest that the OCT-associated PutCT activity does not significantly contribute to in vivo Put synthesis in plants.
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