The catabolism of 3-ketolactose in Agrobacterium.

The existence of a novel enzyme, catalyzing the hydrolysis of 3-ketolactose, is described in the different genetic groups of the genus Agrobacterium. The enzyme differs from the other glycosidases formed by Agrobacterium during growth on lactose. The inducibility of the enzyme could be demonstrated in a strain from biotype 1, but not in a strain from biotype 2. The specific activity of the 3-ketolactose hydrolase is higher in extracts of strains belonging to biotype 2 than to biotype 1. The optimum pH of the enzyme in Tris-HCl buffer is 8.4-8.5. The K(m) value for 3-ketolactose is 2.2 × 10(-2) M. The 3-ketolactose hydrolase is stimulated by Mg(2+) and Mn(2+), and inhibited by Zn(2+). Mercaptoethanol promotes the reaction rate in extracts of strains of biotype 1 but not in extracts of strains of bio-type 2.