Table 3.

Ligand binding and agonist-stimulated cAMP response of the WT hMC4R and S136 mutants

hMC4R	n	NDP-MSH binding		NDP-MSH-stimulated cAMP
		IC50 (nM)	Bmax (% WT)	EC50 (nM)	Rmax (% WT)
WT	4	25.21 ± 0.35	100	1.50 ± 0.60	100
S136A	4	24.84 ± 3.60	108 ± 10	8.06 ± 2.11*	77 ± 8 *
S136C	4	37.37 ± 6.26	116 ± 8	1.07 ± 0.40	37 ± 7†
S136D	4	26.74 ± 8.65	132 ± 6†	2.85 ± 0.54	87 ± 5
S136L	4	21.91 ± 3.79	103 ± 5	10.38 ± 2.30†	129 ± 10*
S136R	3	151.80 ± 7.01‡	97 ± 4	10.73 ± 0.71†	40 ± 5†
S136T	4	43.77 ± 5.64*	116 ± 7	2.45 ± 0.91	116 ± 7
S136Y	4	43.64 ± 9.26	104 ± 6	7.98 ± 1.46†	85 ± 11

^*Significantly different from WT hMC4R, P < 0.05.

^†Significantly different from WT hMC4R, P < 0.01.

^‡Significantly different from WT hMC4R, P < 0.001.

Data shown are the mean ± S.E.M. of the number of experiments indicated. The Rmax (maximal response) was 1225 ± 88 pmol cAMP/10⁶ cells (mean ± S.E.M. of four experiments). IC₅₀ is the concentration of NDP-MSH that is needed to cause 50% inhibition in the binding assay. EC₅₀ is the concentration of NDP-MSH that results in 50% stimulation of the maximal response.