A.H.A.M. van Hoek

Hydrogenosomes: convergent adaptations of mitochondria to anaerobic environments

Hydrogenosomes are membrane-bound organelles that compartmentalise the final steps of energy metabolism in a number of anaerobic eukaryotes. They produce hydrogen and ATP. Here we will review the data, which are relevant for the questions: how did the hydrogenosomes originate, and what was their ancestor? Notably, there is strong evidence that hydrogenosomes evolved several times as adaptations to anaerobic environments. Most likely, hydrogenosomes and mitochondria share a common ancestor, but an unequivocal proof for this hypothesis is difficult because hydrogenosomes lack an organelle genome - with one remarkable exception (Nyctotherus ovalis). In particular, the diversity of extant hydrogenosomes hampers a straightforward analysis of their origins. Nevertheless, it is conceivable to postulate that the common ancestor of mitochondria and hydrogenosomes was a facultative anaerobic organelle that participated in the early radiation of unicellular eukaryotes. Consequently, it is reasonable to assume that both, hydrogenosomes and mitochondria are evolutionary adaptations to anaerobic or aerobic environments, respectively.

Voltage-Dependent Reversal of Anodic Galvanotaxis in Nyctotherus ovalis

Aerobic and anaerobic ciliates swim towards the cathode when they are exposed to a constant DC field. Nyctotherus ovalis from the intestinal tract of cockroaches exhibits a different galvanotactic response: at low strength of the DC field the ciliates orient towards the anode whereas DC fields above 2–4 V/cm cause cathodic swimming. This reversal of the galvanotactic response is not due to backward swimming. Rather the ciliates turn around and orient to the cathode with their anterior pole. Exposure to various cations, chelators, and Ca2‐‐channel inhibitors suggests that Ca2‐‐channels similar to the “long lasting” Ca2‐‐channels of vertebrates are involved in the voltage‐dependent anodic galvanotaxis. Evidence is presented that host‐dependent epigenetic factors can influence the voltage‐threshold for the switch from anodic to cathodic swimming.

Purification and characterization of NADP-dependent glutamate dehydrogenase from the commercial mushroom Agaricus bisporus

The nicotinamide adenine dinucleotide phosphate (NADP)-dependent glutamate dehydrogenase (NADP-GDH) of Agaricus bisporus, a key enzyme in ammonia assimilation, was purified to apparent electrophoretic homogeneity with 27% recovery of the initial activity. The molecular weight of the native enzyme was 330 kDa. The enzyme is probably a hexamer, composed of identical subunits of 48 kDa. The isoelectric point of the enzyme was found at pH 4.8. The N-terminus appeared to be blocked. The enzyme was specific for NADP(H). The Km-values were 2.1, 3.2, 0.074, 27.0, and 0.117mM for ammonia, 2-oxoglutarate, NADPH, L-glutamate, and NADP respectively. The pH optima for the amination and deamination reactions were found to be 7.6 and 9.0, respectively. The temperature optimum was 33°C. The effect of several metabolites on the enzymes activity was tested. Pyruvate, oxaloacetate, ADP, and ATP showed some inhibitory effect. Divalent cations slightly stimulated the aminating reaction. Antibodies raised against the purified enzyme were able to precipitate NADP-GDH activity from a cell-free extract in an anticatalytic immunoprecipitation test. Analysis of a Western blot showed the antibodies to be specific for NADP-GDH.

Symbiotic Associations Between Methanogenic Archaea, Protists and Metazoa: Evolutionary Implications

Long-lasting associations between organisms that belong to different taxa are described as “symbiosis”. They are very complex phenomena and, as we will emphasize in this essay, “symbiosis” can have a meaning beyond an exchange of nutrients and vitamins (Margulis 1976; Rennie 1992). Symbiosis requires a continuous cross-talk between organisms in the form of informational, physiological and behavioural interactions. Additionally, symbiotic associations have their peculiar evolutionary histories, involving co-evolution of host and symbiont (Douglas 1994; Moran and Baumann 1994; Page and Hafner 1996; Werren 1997; Hackstein 1997).