Metabolism of the apoproteins in pulmonary surfactant

Two proteins having nominal molecular weights of 35,000 and 10,000 daltons are found in pulmonary surfactant. Although experiments on their immunological properties suggest that they share antigenic determinants, their metabolic relationship is unknown. To study this question we injected [14C]palmitic acid or L-[3H]leucine into the femoral vein of 59 puppies. We killed the animals 30 min to 68 h after injection and purified surface-active material from the endobronchial lavage fluid. We isolated the 35,000 apoprotein, the 10,000 apoprotein, and the saturated phosphatidylcholines in surfactant and measured their specific activities at various times after injection. We found that the 35,000 apoprotein appears in alveolar surfactant with the same time course as saturated phosphatidylcholine but is cleared more rapidly than is the lipid. The specific activity of the 10,000 apoprotein reaches a maximum after that seen for the 35,000 apoprotein and decays with the same turnover time as that of the lipid. The kinetic data suggest that the 10,000 apoprotein is a metabolic product of the 35,000 apoprotein. They are not consistent with the possibility that the 10,000 apoprotein is an artifact of nonspecific degradation during preparation.