Antiserum was raised against purified human liver α-D-mannosidase B. It precipitated α-mannosidases A and B from solution, demonstrating the close structural resemblance of these 2 forms of acidic α-mannosidase activity. A continuous enzymically active precipitin line with no spurs was obtained when α-mannosidase A and B were placed in adjacent wells on Ouchterlony double-diffusion plates. The antiserum precipitated acidic but not neutral α-mannosidase from an extract of human liver, confirming that the acidic and neutral activities are not closely related. Acidic activity was also precipitated from extracts of human brain, kidney and leucocytes by the antiserum. However, it did not cross-react with bovine acidic α-mannosidase activity or with the activity in human plasma that has an optimum pH of 5.5. The two acidic forms of human liver α-mannosidase, A and B, are immunologically identical but distinct from neutral α-mannosidase and that activity with an optimum pH of 5.5.
