Inhibition of liver surface membrane Na+, K+-adenosine triphosphatase, Mg+2-adenosine triphosphatase and 5′- nucleotidase activities by protoporphyrin: Observations in vitro and in the perfused rat liver

To clarify the pathogenesis of protoporphyrin-induced cholestasis, liver surface membrane enzyme activities were determined after (a) isolated rat liver perfusion with protoporphyrin administered by bolus (1.0 μmol) or bolus plus constant infusion (1.0 μmol + 0.05 μmol/min) and (b) combination of liver surface membrane with protoporphyrin (0.9–53.4 nmol/ml) in vitro. The perfusion studies showed that protoporphyrin significantly inhibited Na+, K+- and Mg+2-adenosine triphosphatase activities. In vitro, these adenosine triphosphatase activities and the 5′-nucleotidase activity were inhibited linearly by protoporphyrin up to a concentration of ~18 μmol/ml; thereafter, enzyme activity was maintained. Greater inhibition of the adenosine triphosphatase activities occurred with protoporphyrin than was reported for chlorpromazine at similar molar concentrations. This effect was independent of the quantity of membrane protein analyzed and was not reversible with a 50: 1 dilution. The inhibitory effect of protoporphyrin on surface membrane enzyme activities was also nonselective. Although the hepatotoxic effects of protoporphyrin may be more generalized, the present data underscore protoporphyrin's toxic interaction with liver surface membranes.