The importance of phosphoinositide turnover in signal transduction has been well documented and the role of phosphoinositide-specific phospholipase C (PI-PLC) has stimulated much progress in defining the PI-PLC isozymes. PI-PLC is known to comprise at least nine isozymes which can be separated into three structurally related classes (PLC-β, PLC-γ, PLC-δ) (Rhee et al. 1989; Kriz et al. 1990). However, the mechanism is not completely understood by which the isozymes are activated upon receptor stimulation to generate the second messengers inositol 1,4,5-trisphosphate (IP3), and 1,2-diacylglycerol (DG). Recent studies reveal that multiple PLC isozymes exist within a single cell (Rhee et al. 1991), suggesting that their respective physiological roles and regulation may be distinct. Receptors which activate PI-PLC after ligand binding act via two established mechanisms. Several lines of evidence indicate that GTP-binding protein(s), putatively called Gp, mediate receptor-coupled phosphoinositide hydrolysis. There are pertussis toxin (PT)-sensitive and insensitive forms of Gp. Although the PT-sensitive Gp has not yet been specified, recently one of the PT-insensitive G proteins, Gq, which couples to PLC-β1 has been isolated (Taylor et al. 1991). A second type of receptor-mediated activation of PI-PLC is catalyzed by receptors with intrinsic protein tyrosine kinase activity such as EGF and PDGF receptors, where the PLC-γ1 isozyme is phosphorylated on tyrosine residues (Rhee et al. 1991). Thus it is conceivable that different PLC isozymes could be coupled to different receptors and involved in expression of different functions.KeywordsTyrosine PhosphorylationHuman PlateletPertussis ToxinPhosphoinositide HydrolysisBovine PlateletThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.