By 30min continuous infusion of [3H]leucine into rats, the specific radioactivities of plasma leucine and tissue-free and tRNA-bound leucine in heart were equal. The specific radioactivity of leucyl-tRNA in heart therefore follows a time course identical with that of plasma leucine soon after the start of infusion. The half-life of cardiac myosin heavy chain (5.5 days) was the same as that reported by other investigators who used the pulse-labelling protocol.
