RELATIONSHIP BETWEEN ASCORBYL RADICAL INTENSITY AND APOPTOSIS-INDUCING ACTIVITY

Ascorbic acid and its related compounds were compared for their ascorb yl radical intensity and apoptosis-inducing activity. Sodium L-ascorba te, L-ascorbic acid, D-isoascorbic acid, sodium 6-beta-O-galactosyl-L- ascorbate and sodium 5,6-benzylidene-L-ascorbate, at the concentration of 1-10 mM, induced apoptotic cell death characterized by cell shrink age, nuclear fragmentation and internucleosomal DNA cleavage in human promyelocytic leukemic HL-60 cells. On the other hand, L-ascorbic acid -2-phosphate magnesium salt and L-ascorbic acid 2-sulfate did not indu ce any of these apoptosis-associated characteristics. ESR measurements revealed that all the active compounds were progressively degraded, p roducing the ascorbyl radical (g=2.0064, hfc=0.17 mT) in culture mediu m, whereas the inactive compounds were stable and did not produce the ascorbyl radical. Cytotoxicity began to appear when the radical intens ity exceeded a certain threshold level. In the presence of N-acetyl-L- cysteine, both ascorbyl radical intensity and apoptosis-inducing activ ity were significantly reduced. These data suggest the possible involv ement of the ascorbyl radical in apoptosis induction by ascorbic acid- related compounds. Exposure of HL-60 cells to ascorbic acid or its act ive derivatives resulted in the rapid elevation of intracellular Ca2concentration, which might serve as the initial signal leading to the cell death pathway.