Effects of lens major intrinsic protein on glycerol permeability and metabolism

Lens Major Intrinsic Protein (MIP) is a member of a family of membrane transport proteins including the Aquaporins and bacterial. glycerol transporters. When expressed in Xenopus oocytes, MIP increased both glycerol permeability and the activity of glycerol kinase. Glycerol permeability (p(Gly)) was 2.3 ± 0.23 x 10^-6 cm sec^-1 with MIP vs. 0.92 ± 0.086 x 10^-6 cm sec^-1 in control oocytes. The p(Gly) of MIP was independent of concentration from 5 x 10^-5 to 5 x 10^-2 M, had a low temperature dependence, and was inhibited approximately 90%, 80% and 58% by 1.0 mM Hg⁺⁺, 0.2 mM DIDS (diisothiocyanodisulfonic stilbene), and 0.1 mM Cu⁺⁺, respectively. MIP-enhanced glycerol phosphorylation, resulting in increased incorporation of glycerol into lipids. This could arise from an increase in the total activity of glycerol kinase, or from an increase in its affinity for glycerol. Based on methods we present to distinguish these mechanisms, MIP increased the maximum rate of phosphorylation by glycerol kinase (0.12 ± 0.03 vs. 0.06 ± 0.01 pmol min^-1 cell^-1) without changing the binding of glycerol to the kinase (K(M) ~ 10 μM).