1. Protocatechuate 4,5-oxygenase, purified 21-fold from extracts of Pseudomonas testosteroni, was examined in the ultracentrifuge and assigned a mol.wt. of about 140000. 2. When diluted, the enzyme rapidly lost activity during catalysis. Inactivation was partially prevented by l-cysteine. 3. With a saturating concentration of protocatechuate (1·36mm), Km for oxygen was 0·303mm. This value is greater than the concentration of oxygen in water saturated with air at 20°. 4. Cell extracts converted protocatechuate into γ-carboxy-γ-hydroxy-α-oxovalerate, which was isolated as its lactone. 5. γ-Carboxy-γ-hydroxy-α-oxovalerate pyruvate-lyase activity was stimulated by Mg2+ ions and mercaptoethanol. Cells grown with p-hydroxybenzoate as carbon source contained higher concentrations of this enzyme than those grown with succinate.

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