Glycerol Inhibits or Uncouples the Plasma Membrane (Ca2++Mg2+)ATPase of Kidney Proximal Tubules Depending on the Ca2+ Concentration

Abstract

In this report it is shown that glycerol (0.5-10% v/v) stimulate the C12-E9-solubilized renal (Ca2++Mg2+)ATPase in the presence of low concentrations of free Ca2+ (<10-6m). At 4% (v/v), the polyol decreases the K0.5 for Ca2+ from 1.15 to 0.22 μM at the high-affinity site, and a very-high-affinity Ca2+ component appears. This component has a K0.5 ≤ 10_9m and its maximal velocity is about one-third that of the fully activated enzyme (at 10–20 μM Ca2+), which is not affected by glycerol (21.1 and 20.2 nmol • mg1•min-1 in the absence and presence of the polyol, respectively). The low-affinity, inhibitory component of the Ca2+ curve (50–1000 μM) is also unaffected by glycerol. With 0.07 μM free Ca2+ and soluble enzyme, the stimulatory effect of glycerol saturates at ≈10% (v/v). In contrast, with 17 μM free Ca2+, glycerol has little effect up to 10% (v/v), and then progressively inhibits ATPase activity. These data indicate that the effect of the polyol is modulated by the occupancy of the high-affinity Ca2+ sites. In native vesicles, the stimulation promoted by low concentrations of glycerol at low concentrations of Ca2+ is accompanied by inhibition of active Ca2+ transport, indicating that, in these conditions, the polyol uncouples ATPase activity and ATP-driven Ca2+ influx. © 1995, Verlag der Zeitschrift für Naturforschung. All rights reserved.