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The Occurrence and Nature of Ornithine Carbamoyltransferase in Senescing Apple Leaf Tissue ¹

^a Department of Horticulture, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801

Ornithine carbamoyltransferase (EC 2.1.3.3) activity was detected in apple (Pyrus malus L.) leaf tissue from early June to November. Total activity remained relatively constant at 4.1 µmoles citrulline produced per hour per 10 cm² until mid-October when it sharply doubled. Following the first frost of the autumn, the enzyme lost about 80% of its former activity. The enzyme from apple leaf exhibited two pH optima, one at pH 8.6 and the other at pH 7.8, indicating the presence of isozymes or two forms of the enzyme. At pH 8.6, a partially-purified enzyme preparation had binding contrasts for its substrates of 6 mM for carbamyl-phosphate and 4.8 mM for ornithine. At pH 7.8, the Km for carbamyl-phosphate was 1.9 mM and the Km for ornithine was 1.22 mM.