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 tr ansporters. When expressed in Xenopus oocytes, MIP increased both glyc erol permeability and the activity of glycerol kinase. Glycerol permea bility (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 con trol 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 50% by 1.0 mM Hg++, 0.2 mM DIDS (diisothiocyanodisulfonic stilben e), and 0.1 mM Cu++, respectively. MIP-enhanced glycerol phosphorylati on, 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 w e present to distinguish these mechanisms, MIP increased the maximum r ate 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 t o the kinase (K-M similar to 10 mu M).