Four different types of alpha-mannosidase activity were shown to occur in several tissues from the rat. There is the Zn2+-dependent enzyme, active at acidic pH, and three enzymes that are active near to neutral pH. 2. The 'neutral' enzymes are activated by Fe2+, Co2+ or Mn2+. 3. Optimum activities for these three enzymes are shown at pH values of 5.2, 6.5 and 7.3. The activity at pH6.5 is the only one evident without metal-ion activation, but activity is enhanced by all three metal ions. The activity at pH 5.2 is seen only in the presence of Fe2+ or Co2+, and the activity at pH7.3 is seen only in the presence of Co2+ or Mn2+ and in a non-chelating buffer medium. 4. The pH6.5-active enzyme is inactivated by EDTA, but activity is restored by excess of metal ion. 5. The enzymes differ markedly in their stability. The pH6.5-active enzyme is very labile and the pH7.3-active enzyme is the most stable. 6. Tissue preparations vary widely in their activity at pH6.5, but where activity is low it can be increased by incubation with one of the activating metal cations. 7. All the enzymes active at neutral pH are inhibited by heavy-metal ions and stabilized to some extent by thiol groups.
