Kushmerick C, Varadaraj K, Mathias R T
Department of Physiology and Biophysics, State University of New York at Stony Brook, Stony Brook, New York 11794-8661, USA.
J Membr Biol. 1998 Jan 1;161(1):9-19. doi: 10.1007/s002329900310.
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 (pGly) 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 pGly 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 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 (KM approximately 10 micron).
晶状体主要内在蛋白（MIP）是膜转运蛋白家族的成员之一，该家族包括水通道蛋白和细菌甘油转运蛋白。当在非洲爪蟾卵母细胞中表达时，MIP可提高甘油通透性和甘油激酶的活性。有MIP时甘油通透性（pGly）为2.3±0.23×10⁻⁶ cm·sec⁻¹，而对照卵母细胞中为0.92±0.086×10⁻⁶ cm·sec⁻¹。MIP的pGly在5×10⁻⁵至5×10⁻² m的浓度范围内与浓度无关，对温度的依赖性较低，分别被1.0 mM Hg²⁺、0.2 mM DIDS（二异硫氰酸二磺基芪）和0.1 mM Cu²⁺抑制约90%、80%和50%。MIP增强了甘油磷酸化作用，导致甘油掺入脂质的量增加。这可能是由于甘油激酶总活性增加，或者是其对甘油的亲和力增加所致。基于我们提出的区分这些机制的方法，MIP提高了甘油激酶的最大磷酸化速率（0.12±0.03对0.06±0.01 pmol·min⁻¹·cell⁻¹），而没有改变甘油与激酶的结合（Km约为10微米）。
