Localizing -SH groups in monoaminergic synaptic vesicles with the mixture of zinc iodide-osmium tetroxide (ZIO).

In monoaminergic granulated vesicles the mixture of zinc iodide-osmium tetroxide (ZIO) stains two compartments: the core and the matrix, the latter being more intensively stained than the core. Rat pineal glands were incubated in 0.1 M N-ethylmaleimide (NEM). It was observed that NEM blocks ZIO reaction both in the matrix and the core, whereas the controls were fully reactive. This finding strongly suggests that ZIO reaction is due to -SH groups and can be correlated with the effect of NEM on uptake processes.