Invertebrate Survival Journal (Apr 2005)
Calcium/calmodulin dependence of nitric oxide synthase from Viviparus ater
Abstract
The calcium ion dependence of soluble and particulate nitric oxyde synthase (NOS) activity fromViviparus ater immunocytes was investigated. At a calcium ion concentration of 2 nM, the NOS activitymeasured by citrulline formation was 27.1 ± 2.2 and 9.3 ± 0.8 pmol/min/106cell for soluble andparticulate NOS, respectively. The increase in free calcium ion concentration to 300 nM increasesenzyme activity to 57.5 ± 4.1 and 23.5 ± 1.2 pmol/min/106cell, respectively. The 50 % activation of thecalcium-dependent activity is 91 and 97 nM Ca2+ for soluble and particulate enzymes. Trifluoperazine,an inhibitor of the calmodulin-dependent enzyme, partially inhibits both activities. Soluble NOS is fivetimes more sensitive than particulate NOS. The behaviour of both activities with three NOS inhibitors(7-nitroindazole, S-methylisothiourea sulphate, diphenyleneiodonium) is very similar, with IC50 valuesthat are not significantly different. The calcium ion dependence of NOS activities, in a range of freecalcium ion variations, which are transiently observed in receptor-stimulated cells, suggests that nitricoxyde in V. ater immunocytes not only has a defensive role but also signalling relevance in crosstalkingbetween immunocytes and other cells.
