A spectrophotometric method for determination of cysteine and related compounds.

The voluminous literature on the methods of cysteine determination has been reviewed by several authors (l-3). Most of the proposed methods will not differentiate between cysteine and cysteine-containing peptides. The Sullivan method with its numerous modifications (4, 5)) though widely used, has not yet, however, evolved to an entirely satisfactory procedure (3). The absorption of cysteine in the ultraviolet region of the spectrum lacks a characteristic peak and is of a relatively low intensity; therefore, its exploitation for analytical purposes (6,7) seems to be of rather limited value. The sensitive spectrophotometric method, based on the increase in absorbency in the 250 rnp region accompanying mercaptide formation with p-chloromercuribenzoate, cannot be specifically applied for cysteine estimation, as this reaction is shared by thiol compounds in general (8). In the course of a study of the inhibitory effect of fluoropyruvic acid on the respiration of microorganisms’ and mitochondrial preparations: it was noted that thiol compounds are able to prevent the inhibitory action of fluoropyruvate. Further investigation revealed that, as a result of interaction between fluoropyruvate and thiol compounds, the -SH groups disappear and equivalent amounts of hydrofluoric acid are liberated. The absorption spectra of the reaction products show well defined peaks in the ultraviolet region. The utilization of this phenomenon for the determination of cysteine and related compounds forms the subject of the present communication.