High-affinity Ca 2+-Mg 2+-ATPase in kidney of euryhaline Gillichthys mirabilis: kinetics, subcellular distribution and effects of salinity

Abstract

1. 1. Two components of Ca 2+-Mg 2+-ATPase are observed in kidneys of G. mirabilis. The high-affinity component has a K 0.5Ca of 0.23μM; the low-affinity activity K 0.5Ca is 90–110μM. The high-affinity activity requires Mg 2+, displays Michaelis-Menten kinetics, has peak activity at 1.2 μM Ca 2+, and is insensitive to ouabain and Na + azide. 2. 2. In subcellular fractions, the high-affinity component segregates with Na +-K +-ATPase and is localized predominantly in BLM. The low-affinity component is broadly distributed among membranous organelles, including brush border, and may be equivalent to alkaline phosphatase. 3. 3. Specific activity of the high-affinity Ca 2+-Mg 2+-ATPase is modestly increased following adaptation of fish to FW, but total renal high-affinity activity is greatest in the hypertrophied kidneys of FW-adapted fish and is least in kidneys of fish adapted to 200% SW. 4. 4. High-affinity Ca 2+-Mg 2+-ATPase may be associated with active Ca 2+ transport or with regulation of intracellular Ca 2+ concentration of tubular cells.