Vitamin C redox reactions in blood of normal and malaria-infected mice studied with isoascorbate as a nonisotopic marker

Abstract

It has been suggested that the host antimalarial response depends in part on phagocyte-derived oxidants and that the parasite itself exerts an oxidative stress on its erythrocytic environment. Intraerythrocytic malaria parasites are particularly susceptible to being damaged by oxidative drugs, several of which are under development as chemotherapeutic agents. Thus the antioxidant status and associated regulatory mechanisms of the blood during malaria infection are of great interest. The important antioxidant ascorbate (AH −) and isoascorbate (IAH −), an isomer that does not occur naturally in animals, were found to have similar redox properties. We therefore assessed the usefulness of IAH − as a marker for studies of AH − handling in vivo and in vitro under normal conditions and in murine malaria infection. DHIA added to whole blood from normal or Plasmodium vinckei- infected mice in vitro was rapidly taken up into blood cells and reduced to IAH −. Intracellular IAH − derived from the exogenous DHIA was released into the plasma by blood cells from malaria-infected mice but not those from normal mice. Uptake and reduction of DHIA had no effect on plasma or cellular levels of AH − under these conditions. IAH − injected IV − into either normal or P. vinckei-infected mice was rapidly cleared in both cases and led to an increase in plasma levels of AH −; this suggested displacement of the latter from some intracellular site, presumably not associated with blood cells. DHIA administered as an intravascular bolus into either normal or malaria-infected mice was rapidly reduced. However, in contrast to the in vitro situation, the concentration of plasma IAH − derived from the injected DHIA was approximately the same in both the infected and control animals. The IAH − so formed disappeared quickly from the plasma. Intravenous injection of DHIA into malaria-infected mice caused a rapid, prolonged increase in the proportion of plasma vitamin C in the form of DHA, whereas in uninfected mice there was a transient decrease in plasma DHA followed by normalisation. The changes in plasma AH − and DHA following IV injection of a single dose of DHA closely paralleled those seen after DHIA administration. These observations indicate that: (i) blood cells from normal and malaria-infected mice take up and reduce DHIA in a similar fashion, but they have different ways of handling the resulting IAH −; (ii) cells other than blood cells are important in the reduction of plasma DHIA and DHA in vivo; (iii) malaria-infected mice are less capable of handling oxidative challenge than normal ones; (iv) in some circumstances IAH − and DHIA may be useful nonisotopic markers for studies of vitamin C handling in vitro and in vivo.