Regulation of human lymphoblast plasma membrane 5'-nucleotidase by zinc.

Abstract

Human lymphoblasts lose >90% of their plasma membrane 5’-nucleotidase activity within 24 h when grown in zinc-deficient (<0.05 ~LM Zn(I1)) culture media prepared with either bovine serum albumin/fatty acids/ transferrin or with extensively dialyzed fetal bovine serum as supplements. The loss of 5’-nucleotidase activity is accompanied by a decrease in substrate affinity. The inactivation of plasma membrane 5’-nucleotidase is temperature dependent and requires the metabolically active cell. When isolated plasma membranes are incubated under the same conditions as the whole cells, they do not lose their 5‘-nucleotidase activity. Supplementation of the zinc-deficient culture media with 10 p~ zinc prevents the loss of 5’-nucleotidase activity. Addition of 10 p~ zinc to zinc-deficient cells or plasma membranes prepared from cells with low 5’nucleotidase activity due to zinc depletion leads to complete recovery of activity within 24 h. The recovery is not prevented by inhibition of protein synthesis. The reactivation of 5’-nucleotidase shows a dependence on temperature which is clearly distinct from the inactivation process. The addition of Co(II), Cu(II), or Cd(I1) to zinc-deficient cultures leads to some activation of 5’nucleotidase, but none of the divalent cations tested were nearly as effective as zinc. The observed changes in 5’-nucleotidase activity were not found in other divalent cation-requiring plasma membrane enzymes (ATPase, alkaline phosphatase). Our findings suggest that 1) plasma membrane 5’-nucleotidase can exist as an inactive apoenzyme and 2) zinc plays a unique role in the expression of plasma membrane 5’-nucleotidase activity.