Benzylpenicillin migrates irreversibly into human erythrocytes.

Benzylpenicillin, 5.2% 14C-enriched, was incubated with human whole blood. The erythrocyte component was washed and exhaustively dialyzed and found to contain 6.3% of the original radioactivity. None of the penicillin was found to bind to the cell membrane. Dialysis of the cell contents indicated that none of the penicillin binds irreversibly to the proteins within the cell. Infrared examination of the contents of the cell leads to the conclusion that benzylpenicillin (a monovalent anion) enters the erythrocyte and is hydrolyzed to benzylpenicilloic acid (a divalent anion). This structural change prevents the antibiotic from migrating out of the cell.