Abstract: The present invention relates to a novel crystalline thio-NADP potassium salt, which is produced by preparing thionicotinamide adenine dinucleotide phosphate (thio-NADP) into the potassium salt thereof and thereafter crystallizing the salt. Conventionally known non-crystalline thio-NADP sodium salt has a strong hygroscopic property to deliquesce on contact with air, of which the storage is therefore difficult. The crystalline thio-NADP potassium salt of the present invention has no hygroscopic property so that it does not deliquesce on contact with air, which can be used as research reagents, diagnostic agents and the like for a long period of time and which can be stored for a long period of time.

Abstract: A group of compounds having the following general formula can be obtained by reacting chromoprotein and organic acid: ##STR1## wherein X is a chromophore, P is a protein, NH.sub.2 is mainly .epsilon.-amino group of lysine, n is a positive integer of 1 to 18, and m is a positive integer in the relation of m.ltoreq.n. This compound group has one to eighteen organic acids bonded thereto, and shows various isoelectric points in accordance with the number of the organic acids, but the respective isoelectric points are maintained constant. If such compounds each having an individual isoelectric point, are used as isoelectric point markers, it is possible to recognize accurately the position of isoelectric point only by a visual operation.