Method for producing complex of biotin-labeled Cypridina (Cypridina noctiluca) luciferase with streptoavidin and method for stabilizing the same

An immunoassay method comprising steps of:

• reacting an analyte with a first complex comprising a biotin-labeled Cypridina luciferase, a polyvalent avidin substance and an antibody capable of recognizing the analyte, to form a second complex, and
• contacting a luciferin with the second complex and measuring a luminescence, wherein:
• a biotin of the biotin-labeled Cypridina luciferase
• is bound at a position corresponding to a lysine residue K180 and/or K203 of SEQ ID NO:
• 1 via a polyethylene glycol moiety represented by formula —(CH2CH2O)n—
• wherein n is an integer of 2 to 50, or
• is bound via a sugar chain with a hydrazide linkage;
• the biotin-labeled Cypridina luciferase is labeled with 2 to 5 biotins;
• and
• the polyvalent avidin substance is selected from the group consisting of avidin, streptavidin and NeutrAvidin.