Quantitative microspectrophotometrical determination of protein thiols and disulfides with 2,2'-dihydroxy-6,6'-dinaphthyldisulfide (DDD). The variety of DDD-staining methods demonstrated on Ehrlich ascites tumor cells.

2,2'-dihydroxy-6,6'-dinaphthyldisulfide (DDD) reacts with both protein thiol groups and with protein disulfides (Nöhammer 1977). By varying the pH of the DDD-reaction, as well as the reaction times, the complex reaction became specific with respect to the histochemical demonstration of protein-SH groups. Furthermore, the application of the histochemical DDD-reaction following quantitative blockade of the protein-SH groups enabled the demonstration of distinctive DDD-reactive disulfides. The specificity and the extent of the different histochemical DDD-staining methods were investigated by comparing macroscopically determined values of the protein-SH-contents, and the contents of the different kinds of disulfides in Ehrlich-ascites-tumor cells (EATC) (Modig 1968; Hofer 1975), with microspectrometrical values determined with the MCN-method of Nöhammer et al. (1981), and with microspectrometrical values measured on EATC after staining with the modified DDD-methods. Also, the method for the histochemical demonstration of protein-SH with DDD after the reduction of the disulfides with thioglycolate was investigated and conditions were found by which the protein-SH content could be determined quantitatively with DDD and Fast blue B after the reduction of the disulfides. With the aid of the MCN-method (Nöhammer et al. 1981), the intracellular disulfide interchange reaction was investigated, leading to pH-dependent changes of the SH-SS-ratio of fixed cells during their incubation in aqueous media. In addition the possibility of protein loss during the long incubation times of the fixed cells in the DDD-solutions was investigated. For the quantitative microscpecrometrical determination of the protein content of EATC the so-called tetrazonium-coupling method, optimized by Nöhammer (1978) and calibrated by Nöhammer et al. (1981), was used.