Serine/threonine protein phosphatases expected to participate in the process of signal transduction, cell movement such as cell division and gene expression (Kinoshita et al., 1990; Healy et al., 1991; Mayer-Jaekel et al., 1993; Mumby & Walter, 1993), are classified into type 1 (PP1), type 2A (PP2A), type 2B and type 2C in mammalian cells. PP1 and PP2A are known to be strongly inhibited by okadaic acid (OA) (Tachibana et al., 1981; Bialojan Takai, 1988), a polyether fatty acid isolated from the marine sponge Halicondria okadai (Haystead et al., 1989). OA is also known to inhibit PP2A at lower concentrations than that to block PP1 in mammalian cells, but does not inhibit the activities of other phosphatase species (Ishihara et al., 1989).

The p-nitrophenyl phosphate (pNPP) splitting activity in the extract obtained from eggs of the sea urchin Hemicentrotus pulcherrimus was found to be inhibited by OA and calyculin A (CLA), potent inhibitors of PP1 and PP2A. OA-sensitive phosphatases are known to catalyse pNPP splitting (Takai & Mieskes, 1991), in the same manner as other OA-insensitive phosphatases.

Four peaks of the pNPP splitting activity were obtained by QAE-Toyopearl chromatography in the extract of sea urchin eggs. In two of these four peaks, pNPP splitting reactions were strongly inhibited by OA and CLA at quite low concentration. High sensitivities of the pNPP splitting reaction to OA and CLA in these two peaks suggest that pNPP splitting results from the reaction catalysed by PP2A. The molecular masses of proteins exhibiting OA-sensitive pNPP splitting activities in these two peaks were found to be about 160 kDa by Superdex 200HR, and were similar to that of mammalian PP2A trimeric holoenzyme. By immunoblot analyses with anti-human PP2A catalytic subunit antibody, an immunoreactive 36 kDa protein was found by SDS-PAGE in a peak of OA-sensitive pNPP splitting activity obtained by QAE-Toyopearl chromatography. Sea urchin eggs have at least two PP2A-like enzymes with similar molecular masses to that of mammalian PP2A, and one of them contains human-type catalytic subunit.