Mannosidase activity

Table II. Effects of Various Metal Ions on mannosidase Activity...

Ovariectomy caused a pronounced fall in the a-D-mannosidase activity of mouse uterus, and the normal level was restored by administration of estrone.28 Although the ovary is not particularly rich in a-D-man-nosidase, the greater proportion of the activity in cattle, pig, and human ovaries was found to be concentrated in the corpus luteum.14,28... 

Results are expressed as fig of p-nitrophenol liberated per g of seed from 6 mM p-nitrophenyl a-D-mannoside in 1 hr at 37° and pH 5. Coarsely ground beans. Defatted sweet-almond meal. A commercial /3-D-glucosidase preparation purified from emulsin. a-D-Mannosidase activity is given per g of purified material. 

Distribution of a-D-Mannosidase Activities" in Sucrose Homogenates of Mammalian Tissues15... 

In mouse liver and kidney and in rat liver, a-D-mannosidase activity appeared to be equally distributed between the two cytoplasmic-granule fractions. With mouse spleen and cancer tissue, a considerable proportion of the enzyme was found free in the cytoplasm. Rat spleen, on the other hand, lacked this cytoplasmic fraction. Inasmuch as the enzyme within the cytoplasmic granules was not fully active in a sucrose homogenate until the membranes had been disintegrated, a-D-mannosidase conforms to the definition of a lysosomal hydrolase. 

One of the most striking indications of the importance of Zn2+ for a-D-mannosidase activity was obtained with preparations that had been inactivated by incubation with EDTA. On addition of an excess of Zn2+ to the assay mixture, complete activity was regained instantaneously, regardless of the extent of prior inactivation. (When the EDTA-inactivated enzyme described in Fig. 4 was assayed in the presence of Zn2+, the points followed the line for the Zn2+-stabilized enzyme.) Again, no other cation that we have examined can replace Zn2+, leaving little doubt as to the identity of the activating cation in the original material. It also follows that EDTA must withdraw Zn2+ from the protein-metal complex. Had EDTA merely formed a... 

Although the a-D-mannosidase activity of several different organs in mammals has been found to alter under a variety of conditions (see Section 11,1 p. 402),11,12-14,28,28 the most striking changes are those produced by sex hormones in the epididymis and uterus.26,28 In rats and mice, a-D-mannosidase activity in the epididymis increases with the age of the animal, up to maturity orchidectomy causes a dramatic fall in the enzyme activity of the adult tissue (up to 100-fold), but activity can be partially restored by the administration of testosterone. Ovariectomy results in a 10-fold fall in the activity of a-D-mannosidase in mouse uterus, and this is completely reversed by estrone. 

One of the most interesting aspects of the changes in a-D-man-nosidase activity known to occur in vivo is the possible relationship with changes in zinc concentration in the tissues. In the case of rat epididymis, changes in enzyme activity have been monitored in parallel with measurements of zinc content.26 There was a positive correlation between the two variables, as both increased with age. The drop in a-D-mannosidase activity resulting from orchidectomy was accompanied by a 4-fold fall in the zinc content of epididymis, but the restoration of enzyme activity produced by subsequent injection of testosterone was not reflected in a detectable rise in the proportion of zinc. 

In zinc-deficient rats, the zinc content of the epididymis was only about half the normal value,28,87 and the level of a-D-mannosidase activity was also little over half the value usually observed.26 However, the zinc concentration of the tissue was still in vast excess over that required for stoichiometric combination with the enzyme protein, calculated on any probable estimate of its specific activity and molecular weight (see Section III,5 p. 433). 

Although a-D-mannosidase appears to be under endocrine control,28,28 its natural function remains obscure. It is probable that the enzyme is important in the catabolism of D-mannose-containing glycoproteins,12,16,88 and, therefore, that the important trace element zinc could be indirectly implicated in this process. It is well known that zinc deficiency has an adverse effect on fertility in the male,89,90 and it could be argued that the changes observed in a-D-mannosidase activity in uterus and epididymis indicate that this enzyme is important in reproduction. It is scarcely justifiable to ascribe a cause-and-effect relationship to the correlations observed between the zinc content and the a-D-mannosidase activity in epididymis. All that can be said with any certainty is that, under certain circumstances, the two variables respond together to a common stimulus. 

In a tissue having a high a-D-mannosidase activity, such as the epididymis, zinc may be needed in great excess to lessen dissociation of the catalytically active protein-zinc complex and the consequent displacement of Zn2+ by other bivalent cations (such as Cd2+) to yield... 

Sample quality is generally assessed by the determination of /J-galactosidase activity. For MPS type IVB, the a-mannosidase activity is chosen as an indicator enzyme. Leukocyte homogenates that are sufficient for at least 20 separate runs are prepared from one source and aliquots are kept frozen. These samples serve as quality controls for each run. Heat-inactivated leukocyte homogenates may serve as a positive control (patient-mimics). 

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