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 O-degrees-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.