Immunological probe of estrogen biosynthesis. Evidence for the 2 beta-hydroxylative pathway in aromatization of androgens

The terminal hydroxylation in placental estrogen biosynthesis from androgens is at the 2 beta position. The 2 beta-hydroxy-19-oxoandrogen derivative collapses nonenzymatically to estrogen and is therefore the proximate precursor of the female hormone. To establish the role of this pathway in biological aromatization, an immunological approach was employed in which an antibody was obtained which recognizes 2 beta-hydroxy-19-oxygenated androgens but not intermediates oxygenated at C-19 only. Binding of the 2 beta-hydroxy-19-oxo intermediate by the antibody stabilizes it so that its nonenzymatic transformation to estrogen is delayed and results in slower estrogen formation. When placental microsomes were incubated with [1,2-3H]androstenedione in the presence of the antibody antiserum, a 50% decrease in [3H]estradiol formation and 3H2O release was observed when compared with identical incubations containing normal rabbit serum alone. This inhibition is blocked when the antibody is inactivated by presaturation with 2 beta, 19-dihydroxyandrostenedione. Precipitation of immunoglobulins from the incubations followed by heating liberated the 2 beta-hydroxy-19-oxo intermediate (30%) from the antibody, and resulted in its nonenzymatic collapse to estrogen with concomitant release of 3H2O. Control normal rabbit serum or blocked antibody incubations did not show a similar increase in [3H]estradiol or 3H2O yields in the precipitate. Heat treatment (90 degrees C) of the antibody but not normal rabbit serum incubations resulted in a similar increase in [3H]estradiol and 3H2O yields. These results are consistent with the hypothesis that the final and rate-determining hydroxylation in aromatization of androgens is at the 2 beta position and that this pathway is the dominant, if not the sole, route of estrogen biosynthesis by placental aromatase. The antibody probe also permits the characterization of aromatization mechanisms in tissues other than the placenta.     

Calcium and pancreatic β-cell function 7. Evidence for cyclic AMP-induced translocation of intracellular calcium

The effect of cyclic AMP on calcium movements in the pancreatic β-cell was evaluated using an experimental approach based on in situ labelling of intracellular organelles of ob/ob-mouse islets with ⁴⁵Ca. Whereas the glucose-stimulated ⁴⁵Ca incorporation by mitochondria and secretory granules was increased under a condition known to reduce cyclic AMP (starvation), raised levels of this nucleotide (addition of 3-isobutyl-1-methylxanthine or N⁶,O^2′-dibutyryl adenosine 3′,5′-cyclic monophosphate) reduced the mitochondrial accumulation of ⁴⁵Ca. Conditions with increased cyclic AMP were associated with a stimulated efflux of ⁴⁵Ca from the secretory granules but not from the mitochondria. The microsomal fraction differed from both the mitochondrial and secretory granule fractions by accumulating more ⁴⁵Ca after the addition of 3-isobutyl-1-methylxanthine. The results suggest that cyclic AMP potentiates glucose-stimulated insulin release by increasing cytoplasmic Ca²⁺ at the expense of the calcium taken up by the organelles of the pancreatic β-cells.     

Modelling biome shifts and tree cover change for 2050 in West Africa

Aim Africa is expected to face severe changes in climatic conditions. Our objectives are: (1) to model trends and the extent of future biome shifts that may occur by 2050, (2) to model a trend in tree cover change, while accounting for human impact, and (3) to evaluate uncertainty in future climate projections. Location West Africa. Methods We modelled the potential future spatial distribution of desert, grassland, savanna, deciduous and evergreen forest in West Africa using six bioclimatic models. Future tree cover change was analysed with generalized additive models (GAMs). We used climate data from 17 general circulation models (GCMs) and included human population density and fire intensity to model tree cover. Consensus projections were derived via weighted averages to: (1) reduce inter‐model variability, and (2) describe trends extracted from different GCM projections. Results The strongest predicted effect of climate change was on desert and grasslands, where the bioclimatic envelope of grassland is projected to expand into the desert by an area of 2 million km². While savannas are predicted to contract in the south (by 54 ± 22 × 10⁴ km²), deciduous and evergreen forest biomes are expected to expand (64 ± 13 × 10⁴ km² and 77 ± 26 × 10⁴ km²). However, uncertainty due to different GCMs was particularly high for the grassland and the evergreen biome shift. Increasing tree cover (1–10%) was projected for large parts of Benin, Burkina Faso, Côte d’Ivoire, Ghana and Togo, but a decrease was projected for coastal areas (1–20%). Furthermore, human impact negatively affected tree cover and partly changed the direction of the projected change from increase to decrease. Main conclusions Considering climate change alone, the model results of potential vegetation (biomes) show a ‘greening’ trend by 2050. However, the modelled effects of human impact suggest future forest degradation. Thus, it is essential to consider both climate change and human impact in order to generate realistic future tree cover projections.