Table 1.

Binding and biological activities of secretin and alanine-replacement analogues. Shown are the K_i values of binding of secretin or each of its analogues to CHO-SecR cells and EC₅₀ values of the intracellular cAMP responses stimulated by these ligands. All values represent the means ± S.E.M. of data from three independent experiments performed in duplicate. Shown also are relative affinity (K_i^Ala/K_i^Sec), relative potency (EC₅₀^Ala/EC₅₀^Sec) and ratio of both for each alanine-replacement analogue compared to data for natural secretin (K_i = 3.4 ± 0.4 nM and EC₅₀ = 0.05 ± 0.01 nM).

Peptides			Binding		cAMP response		Relative affinity/ Relative potency
Amino acid	Position	Substitution	Ki (nM)	Relative affinity	EC50 (nM)	Relative potency
His	1	Ala	>1,000	> 300	55.2 ± 8.7	1104
Ser	2	Ala	14.9 ± 2.4	4.4	0.3 ± 0.1	6	0.7
Asp	3	Ala	>1,000	> 300	3.9 ± 0.6	48
Gly	4	Ala	>1,000	> 300	5.1 ± 0.8	102
Thr	5	Ala	73.4 ± 7.3	21.6	0.2 ± 0.05	4	5.4
Phe	6	Ala	>1,000	> 300	4.6 ± 0.5	92
Thr	7	Ala	>1,000	> 300	9.0 ± 1.5	180
Ser	8	Ala	950.5 ± 84.3	279.6	6.1 ± 1.4	122	2.3
Glu	9	Ala	158.4 ± 12.2	46.6	8.5 ± 1.8	170	0.3
Leu	10	Ala	>1,000	> 300	6.1 ± 1.3	122
Ser	11	Ala	40.8 ± 5.6	12	1.0 ± 0.2	20	0.6
Arg	12	Ala	63.8 ± 7.2	18.8	0.3 ± 0.09	6	3.1
Leu	13	Ala	79.4 ± 8.9	23.4	0.8 ± 0.2	16	1.5
Gln	14	Ala	116.5 ± 13.3	34.3	0.2 ± 0.07	4	8.6
Asp	15	Ala	520.5 ± 46.4	153.1	2.6 ± 0.6	52	2.9
Ser	16	Ala	24.9 ± 4.5	7.3	0.1 ± 0.04	2	3.7
Ala	17a	-	3.4 ± 0.4	1.0	0.05 ± 0.01	1	1
Arg	18	Ala	220.2 ± 26.6	64.8	0.4 ± 0.1	8	8.1
Leu	19	Ala	>1,000	> 300	34.5 ± 5.5	690
Gln	20	Ala	24.6 ± 3.2	7.2	0.2 ± 0.04	4	1.8
Arg	21	Ala	23.6 ± 3.5	6.9	0.2 ± 0.03	4	1.8
Leu	22	Ala	168.3 ± 20.0	49.5	11.7 ± 2.1	234	0.2
Leu	23	Ala	>1,000	> 300	33.2 ± 6.2	664
Gln	24	Ala	26.8 ± 4.4	7.9	0.2 ± 0.05	4	2.0
Gly	25	Ala	10.9 ± 1.4	3.2	0.2 ± 0.06	4	0.8
Leu	26	Ala	67.6 ± 8.4	19.9	1.3 ± 0.2	26	0.8
Val	27	Ala	107.8 ± 15.4	31.7	0.5 ± 0.2	10	3.2

^aThe residue at position 17 of natural secretin is an alanine.