D-Galactosidases

When the accessible concentration range of glycal [A] c Kj and kpg- c khydr as for ) -D-galactosidase from E. coli, this reduces to k (1 + [S]/ Kp,) = (kp /K,0 [A] + khydf. The rate constant k yj, for the addition of water has to be measured separately, by the appearance of the 2-deoxy-o-hexose. With )S-D-galactosidase from E. and yS-D-glucosidase from... 

Group (c), a-D-mannosidase from jack beans and from almonds, and a-D-galactosidase from coffee beans, showed no inactivation. The results with these enzymes can possibly be explained by the formation of a (weak) non-covalent complex in which glycosylation is too slow to cause inactivation within the time period of measurements, or, less likely, rapid hydrolysis of the glycosyl-enzyme intermediate. 

The rapid inactivation of human lysosomal / -D-galactosidase is noteworthy, as the reaction had to be carried out at 2 pH units above its pH optimum (pH 4.0) in order to minimize the spontaneous decomposition of the inhibitor. Similar conditions had to be employed for ) -D-xylosidase from Penicil-lium wortmamii (optimum at pH 3.5), which was studied at pH 7.0. 

The fact that ) -D-galactosidase from Escherichia coli is inactivated more rapidly in the absence of Mg than in its presence can be taken as evidence that the activation of the triazene 38, that is, formation of )5-D-galactosyl-methyldiazonium ion, proceeds without acid catalysis, because Mg is required for the proton-assisted catalysis of yS-D-galactoside hydrolysis by this enzyme.Additional evidence for the absence of acid catalysis in the de-... 

V al-Met-Ser-Asp-T rp-Ala-Ala-His-His-Ala-Gly-Val-)S-D-Glucosidase A (bitter almonds) lle-Thr-Glx-Glx-Val-Phe-Gly-Asp-Ser-(Ala, Asxj, Glx, Pro)-Lys ff-D-Glucosidase (human placenta, lysosomal) -Val-Ala-Ser-Gln-Lys-Asn-Asp-Leu-Asp-Ala-Val-Ala-a-D-Glucosidase (sucrase, rabbit small intestine) -lle-Asp-Met-Asn-Glu-Pro-Asn-a-D-Glucosidase (isomaltase, rabbit small intestine) -Gly-Gly-Gln-lle-Asp-Met-)ff-D-Galactosidase (Escherichia co//) -Ser-Leu-Gly-Asn-Glu-Ser-Gly-His-Gly-Ala-... 

Fic. 12.—Sequence of the Glycosphingolipid Shown in Fig. 11, after Enzymic Degradation with a-D-Galactosidase. (Cleavage points, and the masses of fragment ions of the permethy-lated derivative, are shown.)... 

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