Poikilothermic traits in Mashona mole-rat (Fukomys darlingi). Reality or myth?

The African mole-rats (Bathyergidae, Rodentia) is a mammalian family well known for a variety of ecophysiological adaptations for strictly belowground life. The smallest bathyergid, the hairless naked mole-rat from arid areas in Eastern Africa, is even famous as the only truly poikilothermic mammal. Another bathyergid, the Mashona mole-rat (Fukomys darlingi) from Zimbabwe, is supposed to have strong poikilothermic traits, because it is not able to maintain a stable body temperature at ambient temperatures below 20 °C. This is surprising because, compared to the naked mole-rat, this species, together with all congenerics, is larger, haired, and living in more seasonal environment. In addition, other Fukomys mole-rats show typical mammalian pattern in resting metabolic rates. In our study, we measured resting metabolic rate and body temperature of Mashona mole-rats from Malawi across a gradient of ambient temperatures to test its poikilothermic traits. We found that the adult mass specific resting metabolic rate was 0.76±0.20 ml O₂ g⁻¹ h⁻¹ and body temperature 34.8±1.1 °C in the thermoneutral zone (27–34 °C). Body temperature was stable (33.0±0.5 °C) at ambient temperatures from 10 to 25 °C. We thus cannot confirm poikilothermic traits in this species, at least for its Malawian population. Factors potentially explaining the observed discrepancy in Mashona mole-rat energetics are discussed.     

Ecological consequences of the phylogenetic and physiological diversities of acetogens

Acetogens reduce CO₂ to acetate via the acetyl-CoA pathway and have been classically thought of as obligately anaerobic bacteria. Nearly 100 acetogenic species from 20 different genera have been isolated to date. These isolates are able to use very diverse electron donors and acceptors, and it is thus very likely that the in situ activities of acetogens are very diverse and not restricted to acetogenesis. Since acetogens constitute a very phylogenetically diverse bacteriological group, it should be anticipated that they can inhabit, and have impact on, diverse habitats. Indeed, they have been isolated from a broad range of habitats, including oxic soils and other habitats not generally regarded as suitable for acetogens. Although the ecological impact of acetogens is determined by the in situ manifestation of their physiological potentials, assessing their in situ activities is difficult due to their physiological and phylogenetic diversities. This mini-review will highlight a few of the physiological and ecological realities of acetogens, and will focus on: (i) metabolic diversities and regulation, (ii) phylogenetic diversity and molecular ecology, and (iii) the capacity of acetogens to cope with oxic conditions under both laboratory and in situ conditions. This revised version was published online in August 2006 with corrections to the Cover Date.     

NAD-linked alcohol dehydrogenase 1 regulates methylglyoxal concentration in Candida albicans

We purified a fraction that showed NAD⁺-linked methylglyoxal dehydrogenase activity, directly catalyzing methylglyoxal oxidation to pyruvate, which was significantly increased in glutathione-depleted Candida albicans. It also showed NADH-linked methylglyoxal-reducing activity. The fraction was identified as a NAD⁺-linked alcohol dehydrogenase (ADH1) through mass spectrometric analyses. In ADH1-disruptants of both the wild type and glutathione-depleted cells, the intracellular methylglyoxal concentration increased significantly; defects in growth, differentiation, and virulence were observed; and G2-phase arrest was induced.     

Activities of Antioxidant Enzymes during Senescence of Prunus Armeniaca Leaves

During the period of senescence of apricot leaves changes in photosynthetic pigment contents and in the activities of some antioxidant enzymes (superoxide dismutase, catalase, peroxidase and ascorbate peroxidase) were analysed. Significant changes in pigment contents were, in most cases, correlated with changes in activities of the antioxidant enzymes. Modifications in superoxide dismutase and catalase isoform patterns were also observed during the progression of senescence. Both enzyme activities and isoenzyme patterns proved to be genotype-dependent.     

Dynamics of α-Hb chain binding to its chaperone AHSP depends on heme coordination and redox state

Background

AHSP is an erythroid molecular chaperone of the α-hemoglobin chains (α-Hb). Upon AHSP binding, native ferric α-Hb undergoes an unprecedented structural rearrangement at the heme site giving rise to a 6th coordination bond with His(E7).

Methods

Recombinant AHSP, WT α-Hb:AHSP and α-Hb^HE7Q:AHSP complexes were expressed in Escherichia coli. Thermal denaturation curves were measured by circular dichroism for the isolated α-Hb and bound to AHSP. Kinetics of ligand binding and redox reactions of α-Hb bound to AHSP as well as α-Hb release from the α-Hb:AHSP complex were measured by time-resolved absorption spectroscopy.

Results

AHSP binding to α-Hb is kinetically controlled to prevail over direct binding with β-chains and is also thermodynamically controlled by the α-Hb redox state and not the liganded state of the ferrous α-Hb. The dramatic instability of isolated ferric α-Hb is greatly decreased upon AHSP binding. Removing the bis-histidyl hexacoordination in α-HbH58(E7)Q:AHSP complex reduces the stabilizing effect of AHSP binding. Once the ferric α-Hb is bound to AHSP, the globin can be more easily reduced by several chemical and enzymatic systems compared to α-Hb within the Hb-tetramer.

Conclusion

α-Hb reduction could trigger its release from AHSP toward its final Hb β-chain partner producing functional ferrous Hb-tetramers. This work indicates a preferred kinetic pathway for Hb-synthesis.

General significance

The cellular redox balance in Hb-synthesis should be considered as important as the relative proportional synthesis of both Hb-subunits and their heme cofactor. The in vivo role of AHSP is discussed in the context of the molecular disorders observed in thalassemia.