The sensitivity to photodynamic treatment of three plasma membrane enzymes in R3230AC mammary adenocarcinomas was assessed. The activities of Na+K+-ATPase, Mg2+-ATPase and 5′-nucleotidase in isolated membranes were measured after exposure of membranes to either hematoporphyrin derivative or Photofrin II plus light in vitro or in tumor membranes prepared from animals previously injected with 25 mg/kg Photofrin II and sacrificed at various times prior to exposure to light (in vivo-in vitro protocol). The activities of both Na+K+-ATPase and Mg2+-ATPase were inhibited at equivalent rates by Photofrin II in vitro; inhibition was drug dose and light dose related. For 5′-nucleotidase in vitro, a 10-fold higher porphyrin concentration was required to achieve a similar rate of enzyme inhibition as that for the ion-activated ATPases. Injection of Photofrin II in vivo followed by preparation of tumor plasma membranes, which were subsequently exposed to light in vitro, produced no photosensitization of 5′-nucleotidase activity at any time studied (up to 72 h after Photofrin II administration). Under the same conditions Na+K+-ATPase activity was reduced by 40–60% from 2 to 72 h after drug injection, whereas Mg2+-ATPase activity was inhibited by 10–25% over the same time course. The differential sensitivity of these three enzymes observed in this in vivo-in vitro protocol suggests that each enzyme may possess different characteristics, such as three-dimensional configuration or membrane location, that afford varying susceptibility to porphyrin photosensitization. The data also suggest that photosensitivity-induced damage to these ion-activated plasma membrane ATPases could have deleterious effects on tumor cell survival.
Supported by USPHS Grant CA36856, NIH.