CYTOPROTECTION AGAINST MEROCYANINE 540-SENSITIZED PHOTOINACTIVATION OF THE Na+,K+-ADENOSINE TRIPHOSPHATASE IN LEUKEMIA CELLS: GLUTATHIONE AND SELENOPEROXIDASE INVOLVEMENT

When irradiated with broad-band visible light in the presence of merocyanine 540 (MC540), murine leukemia L1210 cells grown under selenium-deficient conditions (Se(-) cells) accumulated lipid hydroperoxides and lost viability more rapidly than selenium-satisfied controls (Se(+) cells). These findings suggest that cytoprotection against photoperoxidation and photokilling is mediated at least in part by selenoperoxidase (SePX) action. Similar protection against photoinactivation of an intrinsic membrane enzyme, the Na⁺,K⁺-ATPase, has been observed. Thus, irradiation of MC540-sensitized Se(-) cells resulted in an immediate and progressive inactivation of ouabain-sensitive Na⁺, K⁺-ATPase; by contrast, activity loss in Se(+) cells was preceded by a prominent lag. Enzyme photoinactivation in Se(-) cells was inhibited by ebselen, an SePX mimetic, confirming that SePX(s) is (are) involved in natural protection. Desferrioxamine treatment (iron sequestration/inactivation) resulted in higher hydroperoxide levels and slower Na⁺,K⁺-ATPase inactivation during MC540/light exposure, whereas ferric-8-hydroxyquinoline treatment (iron supplementation) had the opposite effect. Thus, iron appears to play an important role in both of these processes. In contrast, photoinactivation of another intrinsic enzyme in L1210 cells, acetylcholinesterase (AChE), was unaffected by selenium or iron manipulation. On the basis of these findings, we propose that lipid peroxidation plays an important role in the photoinactivation of Na⁺,K⁺-ATPase, but not AChE. This is consistent with the fact that Na⁺, K⁺-ATPase's active site lies within the membrane bilayer, whereas AChE's active site lies outside the bilayer.