Multiple Cypridina Luciferase Genes in the Genome of Individual Ostracods, Vargula hilgendorfii (Cypridina hilgendorfii)

Pink-colored Cypridina luciferase from Cypridina (Vargula) hilgendorfii: Oxygen-triggered luminescence reaction following NAD(P)H-FMN flavin reductase reaction

The pink-colored Cypridina luciferase (CypLase∗) from Cypridina (Vargula) hilgendorfii contains an unknown chromophore (CypL∗), derived from Cypridina luciferin (CypL). When CypLase∗ was treated with NAD(P)H-FMN flavin reductase (FRase) and NADH, the luminescence intensity in the reaction mixture increased significantly after gentle tapping. This observation suggests that CypL∗ in CypLase is enzymatically converted to CypL by the reduced flavin (FMNH2) through the FRase reaction, and the resulting complex of CypL and CypLase reacts with O2 to emit light. Therefore, CypL∗ might be a dehydrogenated form of CypL and might serve as another storage form in Cypridina specimens.

Bioluminescence of (R)-Cypridina Luciferin with Cypridina Luciferase

Cypridina luciferin (CypL) is a marine natural product that functions as the luminous substrate for the enzyme Cypridina luciferase (CypLase). CypL has two enantiomers, (R)- and (S)-CypL, due to its one chiral center at the sec-butyl moiety. Previous studies reported that (S)-CypL or racemic CypL with CypLase produced light, but the luminescence of (R)-CypL with CypLase has not been investigated. Here, we examined the luminescence of (R)-CypL, which had undergone chiral separation from the enantiomeric mixture, with a recombinant CypLase. Our luminescence measurements demonstrated that (R)-CypL with CypLase produced light, indicating that (R)-CypL must be considered as the luminous substrate for CypLase, as in the case of (S)-CypL, rather than a competitive inhibitor for CypLase. Additionally, we found that the maximum luminescence intensity from the reaction of (R)-CypL with CypLase was approximately 10 fold lower than that of (S)-CypL with CypLase, but our kinetic analysis of CypLase showed that the Km value of CypLase for (R)-CypL was approximately 3 fold lower than that for (S)-CypL. Furthermore, the chiral high-performance liquid chromatography (HPLC) analysis of the reaction mixture of racemic CypL with CypLase showed that (R)-CypL was consumed more slowly than (S)-CypL. These results indicate that the turnover rate of CypLase for (R)-CypL was lower than that for (S)-CypL, which caused the less efficient luminescence of (R)-CypL with CypLase.