Intracellular degradation of hemoglobin transferred into fibroblasts by fusion with red blood cells

Abstract

Hamster fibroblast protein and rabbit hemoglobin were labelled by incubation of fibroblasts (BHK21) or reticulocytes with [³H]leucine. Alternatively, human or rabbit hemoglobin was labelled by carbamoylation of erythrocytes with K¹⁴CNO. The labelled hemoglobins were introduced into fibroblasts by virus‐mediated fusion between the blood cells and fibroblasts. The hemoglobins became uniformly distributed throughout the cytoplasm. Degradation was assessed from release of acid‐soluble radioactivity into the medium. Radioactivity from [¹⁴C]‐carbamoylhemoglobin was released as carbamoylvaline and homocitrulline, and these compounds were not metabolized or reincorporated by the cells. Intermediate degradation products could not be detected. The degradation of hemoglobin followed first‐order kinetics. The half‐life of both carbamoylated and native rabbit hemoglobin in hamster fibroblasts was 28 h, and the half‐life of carbamoylated human hemoglobin was about 150 h in fibroblasts from hamster (BHK21), mouse (Balb/3T3), and man (MRC 5), corresponding to that of the more stable endogenous proteins. Phenylhydrazine increased the intracellular degradation of carbamoylated human hemoglobin about 13 times, whereas the degradation of endogenous proteins was little affected. Hemoglobin was degraded in homogenates at 31% h⁻¹ at pH 5 and 0.3% h⁻¹ at pH 7.4. Phenylhydrazine increased these rates to 45% h⁻¹ and 9.7% h⁻¹, respectively. Growing hamster fibroblasts, which are brought into quiescence by serum deprivation or by high culture density, increase the degradation of endogenous protein and of hemoglobin in parallel.