Table 1.

Subcellular localization of 4-phosphatase

	Nonstimulated		Thrombin-stimulated
	Cytosol	CSK	Cytosol	CSK
Enzyme activity, [3H]Ins(1,3,4)P3 hydrolyzed pmol/min per ml	17 ± 6 (n = 5)	0	11 ± 4 (n = 5)	8 ± 2 (n = 3)
Densitometry, arbitrary units	1	0	0.76 ± 0.015 (n = 3)	0.15 ± 0.025 (n = 5)
Specific activity, enzyme activity per unit densitometry	17	0	14	53

4-Phosphatase activity was determined in the cytosol and cytoskeleton (CSK) of nonstimulated platelets and platelets thrombin-stimulated for 5 min. Densitometric analysis was performed on 4-phosphatase immunoblots of the same platelet fractions, and the specific 4-phosphatase enzyme activity per unit protein was determined.