Abstract: An improvement in the process for the determination of the concentration of adenosinetriphosphate (ATP) in ATP-converting reactions wherein the concentration is a function of the intensity of light emitted when a sample to be assayed is contacted with a bioluminescent reagent based on D-luciferin, luciferase and magnesium ions or certain other metal ions, the emitted light resulting from ATP and D-luciferin becoming bound to the luciferase, comprises the use of one or more competitive inhibitors such as a D-luciferin analogue, or analogues.

Abstract: A method for the photometric determination of the subunit B of creatine kinase (CK-B) in buffered water solution through reaction with creatine phosphate, ADP and the system luciferase/D-luciferin in the presence of a specific antibody inhibiting the subunit M of creatine kinase, comprises making the determination in the presence of a D-luciferin analog, such as L-luciferin, having a structure similar to D-luciferin and being a competitive inhibitor of the firefly luciferase reaction.Also disclosed is a reagent for carrying out the above method comprising creatine-phosphate, ADP, luciferase, D-luciferin, L-luciferin, CK-M inhibiting antibodies, diadenosinpentaphosphate, SH-reagent, magnesium salt and buffer as well as possibly stabilizers and/or complex forming reagent.