An improved cycling assay for nicotinamide adenine dinucleotide

A new cycling assay for NAD that uses thiazolyl blue as a terminal electron acceptor has been found to offer significant advantages over the more established procedure that employs 2,6-dichlorophenolindophenol. With thiazolyl blue, the cycling assay is linear with NAD at picomole levels, and with time for at least 120 min. In contrast, with 2,6-dichlorophenolindophenol as terminal acceptor, the cycling assay deviates considerably from linearity at picomole levels of NAD, and the reaction rates become linear for shorter periods of time as the level of NAD increases. Data are given which provide a basis for choosing optimal assay conditions using the new thiazolyl blue cycling technique.