TABLE 1.

Effect of inhibitors on the purified immunoproteasomes

26 S proteasomes, purified from rabbit spleens, were incubated with different concentrations of inhibitors for 30 min at 37 °C followed by measurements by activities as described under “Experimental Procedures.” Residual activity was plotted against concentration on semi-log plots (supplemental Fig. S1), which were used to determine IC₅₀ values. (To allow for easy comparison with data on Fig. 2, IC₅₀ values are provided instead of K_i and k₂.)

	Active sites			IC50 ratio
	β5i	β2i	β1i	β2i/β5i	β1i/β5i
	IC50 (μm)			-fold
NC-005	0.044	4.6	10	105	225
NC-005-mvs	1.5	∼140	∼140	∼93	∼93
YU-101	0.26	1.9	4.5	7.3	17.3
YU-101-mvs	1.9	≫100a	≫100b
PR-171	0.00028	0.62	2.42	2321	8643
PR-171-mvs	0.011	5.3	22.5	481	2045
PR-957	0.0102	1.04	6.4	102	627
PR-957-mvs	1.0	53	78	53	78

^a 21 ± 1% inhibition was at 81 μm.

^b 16 ± 2% inhibition was at 81 μm (values are mean ± S.E. of two independent measurements).