Table 1

Effects of point mutations of the histamine H₄ receptor on histamine binding and histamine induced intracellular Ca^2⁺mobilization

Receptor	K_D[³H]Histamine	H₄ ReceptorB_max	EC₅₀Histamine	Efficacy
	nM	pmol/mg protein	nM	%
Wild-type	15.3 ± 1.7	0.92 ± 0.07	21 ± 6	100
Asp⁹⁴ → Ala	N.D.	N.D.	N.D.	N.D.
Asp⁹⁴ → Glu	N.D.	N.D.	N.D.	N.D.
Asp⁹⁴ → Asn	N.D.	N.D.	N.D.	N.D.
Asn¹⁴⁷→Tyr	31.4 ± 3.3	1.16 ± 0.12	32 ± 6	36
Asn¹⁴⁷ → Ala	68.3 ± 4.7	0.64 ± 0.02	156 ± 23	49
Thr¹⁷⁸ → Ala	47.9 ± 6.8	0.63 ± 0.08	91 ± 20	91
Ser¹⁷⁹ →Ala	34.4 ± 5.5	1.25 ± 0.15	95 ± 26	91
Thr¹⁷⁸Ser¹⁷⁹ → Ala Ala	64.2 ± 7.1	1.35 ± 0.14	98 ± 22	91
Glu¹⁸² → Ala	N.D.	N.D.	N.D.	N.D.
Glu¹⁸² → Asp	153.6 ± 15.5	0.54 ± 0.05	571 ± 133	54
Glu¹⁸² → Gln	N.D.	N.D.	N.D.	N.D.
Ser³²⁰→ Phe	76.1 ± 9.5	0.39 ± 0.02	5625 ± 650	199
Ser³²⁰ → Ala	32.1 ± 1.9	0.54 ± 0.02	93 ± 22	163

Receptors were transiently expressed in HEK-293 SFM cells and used for [³H]histamine binding assays and measurement of histamine-induced intracellular Ca^2⁺ mobilization. Data were calculated as the mean ± S.E. of at least three independent experiments. Efficacy is expressed as percentage of the response of the wild type receptor to histamine.

N.D., could not be determined.