Heat-treated smooth muscle tropomyosin.

Gizzard smooth muscle tropomyosin, which is close to 100% gamma beta heterodimer in the native state, was heated to about 100 degrees C, at which temperature the chains are dissociated, followed by reassociation by rapid cooling to 0 degree C. This heat-treated tropomyosin was composed of about 58% heterodimer and 42% of the gamma gamma and beta beta homodimers and had a lower viscosity than that of the native protein, indicating a reduced end-to-end polymerization. Close to 100% heterodimer was regenerated if the heat-treated tropomyosin was subjected to slow cooling from 50 degrees C. However, the viscosity remained low and did not return to the value for untreated tropomyosin, suggesting that the 100 degrees C treatment results in irreversible chemical damage to tropomyosin which affects its end-to-end interaction. Therefore, heat treatment of tropomyosin, a procedure widely used in the preparation of smooth muscle and non-muscle tropomyosins, may result in tropomyosin with a different heterodimer/homodimer distribution and different properties from those of the native protein and should be used with caution.