Acidic alpha-mannosidase (EC, optimum pH 4.25, is absent from the plasma of Angus calves with mannosidosis, and the residual alpha-mannosidase activity has an optimum pH of 5.5, intermediate between that of the acidic and neutral alpha-mannosidases. This ‘intermediate’ alpha-mannosidase differs from the acidic form in its kinetic properties, its lack of marked inhibition by EDTA and its thermolability at 55 degrees C and physiological pH. Isoelectric focusing and ion-exchange chromatography show that it exists in at least two forms. The presence of a secondary peak at pH 5.5 in the pH/activity profile of normal plasma and the effect of heating at 55 degrees C indicate that such a form is present in normal plasma. The residual activity in the plasma of a calf with mannosidosis is therefore probably not the product of the defective gene. A differential assay, based on their different stabilities at 55 degrees C, has been developed for measuring the acidic and intermediate alpha-mannosidases in plasma. There was no correlation between the concentrations of the two enzymes in the plasma of Angus cows heterozygous for mannosidosis or in the plasma of normal animals. This precludes the use of the intermediate form as a reference enzyme for the acidic activity in a test for heterozygosity for mannosidosis based on the gene-dosage phenomenon. The concentrations of the intermediate activity were comparable in normal animals and animals homozygous or heterozygous for mannosidosis.

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