By using a method that labels sulfhydryl-containing proteins in situ, we (6,14,15). It reacts selectively with the thiol anion of sulfhydryls, crosses biological membranes, and does not significantly alter the molecular weight of proteins (6,14,15).We have used mB to label the sulfhydryl-containing proteins of the outer membrane of Neissena gonorrhoeae. By labeling during growth, we have eliminated the effect of oxidative changes that occur during harvesting and preparation of fractions, thereby detecting the sulfhydryl status of proteins in situ.A nonpiliated, opaque colonial variant of N. gonorrhoeae 1384 (22, 23) was used for all studies. Growth conditions were as previously described (23) (Pharmacia, Piscataway, N.J.) with 0.9 mM dithiothreitol for 30 min and 5.4 mM mB for 20 min. The treated standards were diluted into sample buffer with mercaptoethanol (ME).Electrophoretic analysis of outer membranes from labeled samples revealed multiple fluorescent bands, with the two major fluorescent bands having apparent molecular masses of 41 and 31.5 kDa (Fig. 1A and B). On the basis of its intense fluorescence, the protein at 41 kDa is the major sulfhydryl-containing protein in the outer membrane. Gels stained for protein also showed the 41-kDa protein to be a major protein band in labeled fractions (Fig. 1D, lanes 3 and 5), with an intensity equivalent to that of PIII as seen in outer membrane fractions prepared by conventional methods. However, unlabeled fractions that were not reduced by ME showed no evidence of this protein (Fig. 1D, lanes 1 and 4). When unlabeled samples were reduced, only a lightly staining protein band at 41 kDa was seen (Fig. 1C, lanes 1 and 4). Samples obtained from different experiments (Fig. 1, lanes 1 to 3 versus lanes 4 and 5) confirmed the differences in the 41-kDa protein between labeled and unlabeled samples. Although there was some variation in the intensity of the 41-kDa protein between the two unlabeled, reduced samples, in both cases there was significantly less protein than in labeled samples.There are several possible reasons for the difference between the labeled and unlabeled samples. A likely explanation, given the nature of sulfhydryl compounds, the nature of the reaction with mB, and the effect of ME on the 41-kDa protein in the unlabeled samples, is that in the unlabeled samples artifacts of distribution occur during preparation of the fractions and result in decreases of the 41-kDa protein in the outer membrane; i.e., the protein undergoes autoxidation, with a change in conformation and loss from the membrane, whereas in labeled samples the 41-kDa protein is stable because of its reaction with mB. Another possibility is