The spin-label approach to labeling membrane protein sulfhydryl groups.

Labeling experiments with viable membrane preparations are optimally undertaken where conditions are extremely mild (minimal excess of modification reagent, very short reaction times) in order to reduce the possibilities of introducing artifacts (cell aging and death, denaturation, etc.). Furthermore, specificity of labeling is a critical requirement where the system is complex and contains many proteins. The spin label IV described above offers several advantages over previously employed reagents. By virtue of its reversibility and ease of quantitation, one may repetitively label and remove label from the same membrane preparation in order to check reproducibility while using the same sample as a control.