Malate Synthesis in Crassulacean Leaves. I. The Distribution of C14 in Malate of Leaves Exposed to C14O2 in the Dark.

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Abstract
The distribution of radioactivity in the malic acid synthesized by mature leaves of Kalanchoe crenata and Bryophyllum calycinum during exposure to C14O2 in the dark was determined by a combination of biological and chemical degradation techniques. The label was confined to the carboxyl groups with 1/3 in C-1 and 2/3 in C-4. This distribution was independent of time of exposure to C14O2 (from 4 sec. to 21 hrs.) and of temperature (in the range 10[degree] to 23[degree]C). It also remained unaltered during subsequent exposure of the leaves to light or darkness in the absence of C14O2. The known mechanisms for the incorporation of CO2 in malate and for the randomization of the carboxyl groups of this acid are discussed and found inadequate to account for the observed results. A new mechanism which involves the incorporation of label in the malate precursor through the prior carboxylation of ribulose diphosphate is proposed, and its implications are briefly considered.