Protein synthesis in rat lung. Measurements in vivo based on leucyl-tRNA and rapidly turning-over procollagen I

Abstract

The relationships of the specific radioactivities of leucine in serum, leucine acylated to tRNA and leucine in procollagen I, procollagen III and total protein in lungs of unanesthetized young male rats in vivo were assessed as a function of time during constant i.v. infusion of radiolabeled leucine. The specific radioactivity of free leucine in plasma reached a steady-state plateau value within 30 min of initiation of [3H]leucine infusion. Leucine acylated to tRNA isolated from lungs had the same specific radioactivity as free serum leucine. Leucine in procollagen I rapidly achieved a specific radioactivity equal to that of serum leucine and leucyl-tRNA, indicating that serum leucine and leucyl-tRNA isolated from total lung were in rapid equilibrium with the precursor leucine pool for procollagen I synthesis. On the basis of leucyl-tRNA or free serum leucine as the precursor, half-times of fractional conversion of procollagen I and III were calculated as 9 and 38 min, respectively. The incorporation of leucine into mixed lung proteins calculated from the tracer studies was 6.8 .mu.mol/day for the first 30 min of the infusion, after which the calculated rate increased to 15.0 .mu.mol/day. This apparent increase correlated with the appearance of rapidly labeled plasma proteins trapped in the lungs. On the basis of sort infusions lasting 30 min or less, followed by vascular perfusion of the lung, the average fractional synthesis rate of mixed pulmonary proteins in young male rats was 20% per day.