Measurement of protein structure change in active muscle by hydrogen-tritium exchange.

A hydrogen-tritium exchange method was developed to study protein structure changes at the molecular level in active muscle. Skinned rabbit psoas fibers mounted on a specially designed holder were selectively tritium labeled at peptide group NH sites that change from a highly protected form in rigor to an easily exchangeable, essentially random coil condition when muscle is activated. The number of sites found to show this behavior varies linearly with thick filament-thin filament overlap, and would correspond to 83 amino acids per myosin molecule in the muscle, although the experiments do not yet place these sites in any given protein. Half of the sensitive sites respond to relaxing conditions as well to activation.