Abstract: There is disclosed a calcitonin derivative and its salt, comprising an amino acid residue at the 1-position of calcitonin which is a cyclic amino acid residue represented by the formula: ##STR1## wherein X represents a sulfur atom or a methylene group; and n represents an integer of 0 or 1. and an amino acid residue at the 7-position which is a cysteine residue which may be substituted with an appropriate protective group.

Abstract: A method and composition for use therein for measuring the activity of creatine kinase (hereinafter referred to as "CK") having good storage stability is disclosed. More particularly, a composition for measurement of CK activity which comprises as main components phosphoglycerate kinase (hereinafter referred to a "PGK") which catalyzes the following reaction 2 and glyceraldehyde-3-phosphate dehydrogenase (hereinafter referred to as "GAPDH") which catalyzes the following reaction 3. Reaction 1 is catalyzed by the creatine kinase.Reaction 1Creatinephosphate+Adenosine-5'-diphosphate (hereinafter referred to as "ADP").revreaction.Creatine+Adenosine-5'-triphosphate (hereinafter referred to as "ATP")Reaction 2ATP+3-Phosphoglycerate.revreaction.ADP+1,3-DiphosphoglycerateReaction 31,3-Diphosphoglycerate+Reduce .beta.-nicotinamideadenine dinucleotide (hereinafter referred to as "NADH").revreaction.Glyceraldehyde-3-phosphate+.beta.