1. Malyl-CoA lyase was found in high activity in extracts of Pseudomonas AM1, Pseudomonas MA, Pseudomonas MS, Hyphomicrobium X and Methylosinus trichosporium. 2. The enzyme cleaves (2S)-malyl-CoA into equimolar amounts of acetyl-CoA and glyoxylate in the presence of Mg2+. 3. The specific activity of malyl-CoA lyase was several-fold higher in Pseudomonas AM1 when grown on C1 compounds than when grown on C2, C3 or C4 compounds. This suggests that the enzyme plays a specially important role in C1 metabolism. 4. It is suggested that its role in C1 metabolism, in organisms utilizing the serine pathway, is to provide the glyoxylate necessary to sustain operation of this pathway. 5. The activity of malyl-CoA lyase in extracts of Pseudomonas MA, Pseudomonas MS and Hyphomicrobium X is 27–50 times higher than the activity of ATP- and CoA-dependent cleavage of malate, suggesting that the latter activity may be due to coupling of two enzymes, malate thiokinase and malyl-CoA lyase. 6. Methane-grown Pseudomonas methanica and Methylococcus capsulatus, which are not known to use the serine pathway, possess appreciable amounts of malyl-CoA lyase. Instead of being used primarily for carbon assimilation, the enzyme may here serve as a route to glycine during biosynthesis of purines and proteins.

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