Glutathione Turnover in the Kidney; Considerations Relating to the γ-Glutamyl Cycle and the Transport of Amino Acids

The overall turnover of glutathione in kidney and liver was determined in studies in which L-[¹⁴C]glutamate was administered to mice. Turnover was much more rapid (about 5 times greater) in kidney than in liver. Studies were also carried out in which 5-oxo-L-[¹⁴C]proline was administered; the first order rate constants for glutathione synthesis from 5-oxoproline in liver and kidney were not far from those found for synthesis of glutathione from glutamate in these tissues. The findings are in accord with the fact that the activities of the enzymes of the γ-glutamyl cycle are much higher in kidney than in liver. The findings of high turnover of glutathione in kidney and the rapid utilization of 5-oxoproline by this organ for glutathione synthesis are consistent with the function of the γ-glutamyl cycle in vivo and the proposed role of γ-glutamyl derivatives in amino-acid transport.     

The guanylate kinase domain of the beta-subunit of voltage-gated calcium channels suffices to modulate gating

Inactivation of voltage-gated calcium channels is crucial for the spatiotemporal coordination of calcium signals and prevention of toxic calcium buildup. Only one member of the highly conserved family of calcium channel β-subunits—Ca_Vβ—inhibits inactivation. This unique property has been attributed to short variable regions of the protein; however, here we report that this inhibition actually is conferred by a conserved guanylate kinase (GK) domain and, moreover, that this domain alone recapitulates Ca_Vβ-mediated modulation of channel activation. We expressed and refolded the GK domain of Ca_Vβ_2a, the unique variant that inhibits inactivation, and of Ca_Vβ_1b, an isoform that facilitates it. The refolded domains of both Ca_Vβ variants were found to inhibit inactivation of Ca_V2.3 channels expressed in Xenopus laevis oocytes. These findings suggest that the GK domain endows calcium channels with a brake restraining voltage-dependent inactivation, and thus facilitation of inactivation by full-length Ca_Vβ requires additional structural determinants to antagonize the GK effect. We found that Ca_Vβ can switch the inactivation phenotype conferred to Ca_V2.3 from slow to fast after posttranslational modifications during channel biogenesis. Our findings provide a framework within which to understand the modulation of inactivation and a new functional map of Ca_Vβ in which the GK domain regulates channel gating and the other conserved domain (Src homology 3) may couple calcium channels to other signaling pathways.     

Evaluation of interleukin-6 and its soluble receptor components sIL-6R and sgp130 as markers of inflammation in inflammatory bowel diseases

Purpose

Interleukin-6 (IL-6) production and signalling are increased in the inflamed mucosa in inflammatory bowel diseases (IBD). As published serum levels of IL-6 and its soluble receptors sIL-6R and sgp130 in IBD are from small cohorts and partly contradictory, we systematically evaluated IL-6, sIL-6R and sgp130 levels as markers of disease activity in Crohn’s disease (CD) and ulcerative colitis (UC).

Methods

Consecutive adult outpatients with confirmed CD or UC were included, and their disease activity and medication were monitored. Serum from 212 CD patients (815 measurements) and 166 UC patients (514 measurements) was analysed, and 100 age-matched healthy blood donors were used as controls.

Results

IL-6 serum levels were significantly elevated in active versus inactive CD and UC, also compared with healthy controls. However, only a fraction of IBD patients showed increased serum IL-6. IL-6 levels ranged up to 32.7 ng/mL in active CD (>?5000-fold higher than in controls), but also up to 6.9 ng/mL in inactive CD. Increases in active UC (up to 195 pg/mL) and inactive UC (up to 27 pg/mL) were less pronounced. Associations between IL-6 serum levels and C-reactive protein concentrations as well as leukocyte and thrombocyte counts were observed. Median sIL-6R and sgp130 levels were only increased by up to 15%, which was considered of no diagnostic significance.

Conclusions

Only a minority of IBD patients shows elevated IL-6 serum levels. However, in these patients, IL-6 is strongly associated with disease activity. Its soluble receptors sIL-6R and sgp130 do not appear useful as biomarkers in IBD.