Conversion of Serum Proteins into Tissue Proteins. ∗

Abstract

Summary

1. Incorporation of S³⁵ and C¹⁴ from injected doubly labeled rat serum albumin or globulin into the tissue proteins of rats was investigated. The ratio S³⁵/C¹⁴ in tissue proteins varies very little during 9 days after injection of the doubly labeled serum proteins. 2. The ratio S³⁵/C¹⁴ in tisues is approximately equal to (S³⁵ _P/C¹⁴ _P) X (S_T/S_P) × Q_c where the first term is the ratio of the 2 isotopes in the injected plasma protein, S_T and S_P the per cent of total sulfur in the analyzed tissue protein and the injected plasma protein, respectively, and Q_c is approximately 1.5. 3. The increase in S³⁵/C¹⁴ ratio (Q_c > 1) indicates impoverishment of C¹⁴ relative to S³⁵ and is attributed to utilization of C¹⁴-amino acids for other metabolic pathways. 4. In rats injected with serum albumin-S³⁵, I¹³¹ the ratio S³⁵/I¹³¹ increases during 9 days from 50 to 100 times. 5. Our results indicate that conversion of plasma proteins into tissue proteins involves breakdown to amino acids or very small peptide fragments. There is no indication for the transfer of large peptide fragments from plasma protein to tissue protein molecules.