Cystathionase decreased in activity during sporulation of Bacillus subtilis. The experimental data obtained from the extract of vegetative cells and sporulating cells suggest that the decrease in cystathionase activity was caused by a heat-labile protease in cells. The extract from sporulating cells was applied to a DEAE-cellulose column and a Sephadex G-75 column. In both chromatograms Fraction I having azocasein-hydrolyzing activity (protease B) showed cystathionase-inactivating activity. Cystathionase was inactivated by protease B in the absence of pyridoxal 5'-phosphate (PLP) but its activity was maintained in the presence of PLP in vitro condition. During sporulation cystathionase level in cells decreased after t_1.5 stage, and the intracellular level of phosphorylated vitamin B₆ decreased after t₂ stage. On the other hand, the production of intracellular protease B commenced at t₀ stage. From these data it is suggested that cystathionase activity in vegetative cells of B. subtilis is maintained by PLP from to t₀ to t_1.5 stage, in spite of the presence of protease B in cells. The inactivation of cystathionase by protease B may begin after intracellular phosphorylated vitamin B₆ decreased rapidly at t₂ stage.

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