Host urokinase-type plasminogen activator participates in the release of malaria merozoites from infected erythrocytes.

Malaria infection of red blood cells is associated with plasminogen activation. Surface immunofluorescence and immunoprecipitation experiments, using specific polyclonal and monoclonal antibodies raised against human urokinase, demonstrate that this activity is due to the binding of host urokinase-type plasminogen activator to the surface of erythrocytes infected by mature forms of Plasmodium falciparum malaria parasites. Depletion of urokinase from the culture medium leads to the inhibition of merozoite release and the accumulation of segmenter-infected erythrocytes; this inhibition is reversed by the addition of human single-chain or two-chain urokinase. These findings are consistent with host urokinase being involved in the process of merozoite release from the red blood cell.

Schizosaccharomyces pombe mitochondria: morphological, respiratory and protein import characteristics.

A technique is described for the isolation and purification of intact, respiratory-competent mitochondria from Schizosaccharomyces pombe. The purified mitochondria are capable of oxidizing NADH and succinate as respiratory substrates, indicating the presence of succinate dehydrogenase and an NADH dehydrogenase located on the outer surface of the inner membrane. Mitochondria display good respiratory control with an ADP/O ratio of < 2. Respiratory activity is linearly dependent upon the redox poise of the quinone pool, suggesting the presence of an unbranched respiratory pathway to molecular oxygen. Immunogold labelling using antisera raised against mitochondrial HSP70 proteins (SSP1, SSC1 and PHSP1) from three different species, namely S. pombe, Saccharomyces cerevisiae and the plant Pisum sativum respectively, has been used to investigate the presence and ultrastructure of the mitochondria isolated by this procedure. The immunocytochemistry was carried out using cells containing wild-type levels of SSP1 protein and cells over-expressing the protein. These results also demonstrate the capacity of mitochondria to import increased levels of protein in vivo. In vitro import experiments using COXIV-DHFR indicate that purified S. pombe mitochondria can efficiently import this precursor, and that protein translocation is dependent upon an oxidizable substrate and a membrane potential.

Role of nonohmicity in the regulation of electron transport in plant mitochondria.

The relationship between the respiratory rate and the membrane ionic current on the protonmotive force has been investigated in percoll purified potato mitochondria. The dependence of the membrane ionic current on the membrane potential was monitored using a methyltriphenylphosphonium-sensitive electrode and determining the maximal net rate of depolarization following the addition of a respiratory inhibitor. We have confirmed that a nonohmic relationship exists between the ionic conductance and membrane potential. Addition of ATPase inhibitors markedly increased the initial rate of dissipation suggesting that in their absence the dissipation rate induced by respiratory inhibitors is partially offset by H(+)-efflux due to the hydrolysis of endogenous ATP. This was corroborated by direct measurement of endogenous ATP levels which decreased significantly following dissipation of the membrane potential. Results are discussed in terms of the regulation of electron transport in plant mitochondria in vivo.