Permeation of a spin-label phosphate into the human erythrocyte

The reduction of spin-labels by human erythrocytes can be used to follow their penetration into these cells. The neutral spin-label alcohol Tempol (4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxyl) diffuses through the membrane very quickly. The membrane is virtually impermeable to the positively charged spin-label Tempo-choline (N,N-dimethyl-N-(2',2',6',6'-tetramethyl-4'-piperidinyl-1-oxyl)-2-hydroxyethylammonium chloride). The negatively charged spin-label Tempo phosphate (4-phospho-2,2,6,6-tetramethylpiperidinyl-l-oxyl) is reduced at 37 degrees, with a half-time of about 1 hr. The reduction occurs internally following the rate-limiting transport of the label across the erythrocyte membrane. Reduction of this spin-label is greatly diminished by the specific inhibitor of anion transport, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS). The rate of transport depends strongly on the transmembrane electrical potential.