o-Diphenol oxidase activity of molluscan hemocyanins

Diphenoloxidase activities of two molluscan hemocyanins, isolated from the marine snails Rapana venosa and garden snails Helix vulgaris were studied using o-diphenol and l-Dopa as substrates. The dimers of H. vulgaris Hc show both, diphenol (Km = 2.86 mM and Kcat = 4.48) and l-Dopa activity due to a more open active sites of the enzyme and better access of the substrates. The Km value of molluscan H. vulgaris Hc is very close to those of Helix pomatia and Sepia officinalis Hcs, but several times higher compared to those of Rapana and Octopus Hcs. Also HvH has a very high enzyme activity compared with other molluscan Hcs. Kinetic measurements with native RvH and both structural subunits, RvH1 and RvH2, show that RvH and only one structural subunit, RvH2, exhibited only o-diphenol activity, but no l-Dopa oxidizing activity.