High affinity (Ca 2+ + Mg 2+)-ATPase in adult brain synaptosomes of the bertha armyworm, Mamestra configurata Wlk.

Abstract

Ca 2+ + Mg 2+)-ATPase in synaptosomes prepared from the adult brain of Mamestra configurata was strictly dependent on ATP and had a single, high-affinity binding site for ATP ( K m = 122 μM). The enzyme was stimulated by Mg 2+ ( K m = 85 μM) and was maximally active in the presence of 40 mM K +. The enzyme was insensitive to ouabain, an inhibitor of (Na + + K +)-ATPase. Moth brain synaptosomal (Ca 2+ + Mg 2+)-ATPase was stimulated by low concentrations of Ca free 2+ and appeared to have two kinetically distinct binding sites for calcium ion, one with a very high affinity for Ca free 2+ at nanomolar concentrations and the other with a lower affinity for Ca free 2+ at micromolar concentrations. The lower-affinity binding site of the moth brain synaptosomal (Ca 2+ + Mg 2+)-ATPase appears to correspond to the Ca 2+ binding site of (Ca 2+ + Mg 2+)-ATPase in mammalian brain synaptosomes. The high-affinity binding site of the moth brain enzyme for Ca free 2+ ( K m = 0.11 nM) has not been described previously in the literature and conceivably may represent a unique class of high-affinity binding sites for calcium in insect brain. Moth brain (Ca 2+ + Mg 2+)-ATPase displays the same high affinity for Ca free 2+ ( K m = 0.11 nm) as described earlier for ATP-dependent Ca 2+ transport ( K m = 0.1–0.2 nM) in synaptosome membrane vesicles, indicating that the two processes are functionally coupled in the synaptic membrane and contribute to the maintenance of calcium homeostasis in the brain of M. configurata.