Spectrophotometric assay of nicotinic acid in blood. Assessment of its daily profile in humans.

A sensitive, simple, and specific spectrophotometric method is developed for estimating nicotinic acid in blood. The method depends upon reducing nicotinic acid by Zn/HCl to piperidine-3-carboxylic acid. The latter is complexed with CS2 and ammoniacal CuSO4 to form the yellow Cu-dithiocarbamate complex. The color is measured at 445 nm, obeys Beer's law (20--900 microgram/ml), and remains stable for more than 4 h. Efficiency of recoveries from standards and blood samples was tested and proved satisfactory at different levels of concentrations. The method shows good recovery (99.66%) and reasonable standard deviation (+/- 0.29%) in comparison with the cyanogen bromide method (98.3% and +/- 0.73%, respectively). The results obtained show a natural gradual decline in blood nicotinic acid level during the course of the day.