The serendipitous origin of chordate secretin peptide family members

Background  

The secretin family is a pleotropic group of brain-gut peptides with affinity for class 2 G-protein coupled receptors (secretin family GPCRs) proposed to have emerged early in the metazoan radiation via gene or genome duplications. In human, 10 members exist and sequence and functional homologues and ligand-receptor pairs have been characterised in representatives of most vertebrate classes. Secretin-like family GPCR homologues have also been isolated in non-vertebrate genomes however their corresponding ligands have not been convincingly identified and their evolution remains enigmatic.     

Demonstration of a new mammalian isoleucine catabolic pathway yielding an Rseries of metabolites.

1. Normal human urine contains small amounts (less than 4 mg/g of creatinine) of 2-ethylhydracrylic acid, formed, we believe, by a previously undisclosed endogenous catabolic pathway for the oxidation of a newly described series of R metabolites of isoleucine. 2. Urinary excretion of 2-ethylhydracrylic acid is variably increased in defects of isoleucine oxidation at distal steps in the catabolic pathway (3-oxoacyl-CoA thiolase deficiency and methylmalonyl-CoA mutase deficiency) and is diminished when proximal steps of the oxidative pathway are blocked as in branched-chain oxo acid decarboxylase deficiency ('maple-syrup-urine' disease). 3. Precursors of R-pathway metabolites [R(-)-2-methylbutyrate and 2-ethylacrylate ] lead to increased 2-ethylhydracrylate excretion in the mammal(rat, rabbit and dog); the corresponding S metabolites [S(+)-2-methylbutyric acid and tiglic acid ], when given in equimolar amounts, have little effect on its excretion, suggesting that little or no interconversion between S and R metabolites occurs in vivo. 4. Studies with 2H-labelled precursors indicate that conversion of R 2-methylbutyrate into 2-ethylhydracrylic acid occurs by a direct pathway (apparently via 2-ethylacrylic acid). 5. The further oxidation of 2-ethylhydracrylic acid to ethylmalonic acid was demonstrated, and may be analogous to S-metabolite oxidation via methyl malonate. 6. Valine metabolites do not interact with the R=isoleucine pathway under the conditions of these experiments in vivo.     

Structural and biochemical characterization of the therapeutic Anabaena variabilis phenylalanine ammonia lyase

We have recently observed promising success in a mouse model for treating the metabolic disorder phenylketonuria with phenylalanine ammonia lyase (PAL) from Rhodosporidium toruloides and Anabaena variabilis. Both molecules, however, required further optimization in order to overcome problems with protease susceptibility, thermal stability, and aggregation. Previously, we optimized PAL from R. toruloides, and in this case we reduced aggregation of the A. variabilis PAL by mutating two surface cysteine residues (C503 and C565) to serines. Additionally, we report the structural and biochemical characterization of the A. variabilis PAL C503S/C565S double mutant and carefully compare this molecule with the R. toruloides engineered PAL molecule. Unlike previously published PAL structures, significant electron density is observed for the two active-site loops in the A. variabilis C503S/C565S double mutant, yielding a complete view of the active site. Docking studies and N-hydroxysuccinimide-biotin binding studies support a proposed mechanism in which the amino group of the phenylalanine substrate is attacked directly by the 4-methylidene-imidazole-5-one prosthetic group. We propose a helix-to-loop conformational switch in the helices flanking the inner active-site loop that regulates accessibility of the active site. Differences in loop stability among PAL homologs may explain the observed variation in enzyme efficiency, despite the highly conserved structure of the active site. A. variabilis C503S/C565S PAL is shown to be both more thermally stable and more resistant to proteolytic cleavage than R. toruloides PAL. Additional increases in thermal stability and protease resistance upon ligand binding may be due to enhanced interactions among the residues of the active site, possibly locking the active-site structure in place and stabilizing the tetramer. Examination of the A. variabilis C503S/C565S PAL structure, combined with analysis of its physical properties, provides a structural basis for further engineering of residues that could result in a better therapeutic molecule.     

Topology of membrane exposure in the renal cortex slice. Studies of glutathione and maltose cleavage

We measured glycine release from ([2-3H]glycine)-labelled GSH and glucose formation from maltose incubated with rat kidney whole cortex homogenate, thin cortex slices or collagenase-treated tubule fragments. Liberation of glycine was inhibited (74-83%) by serine borate (20 mM), indicating a gamma-glutamyltransferase-dependent hydrolysis of GSH. In whole cortex homogenate, the GSH cleavage activity was 17.4 +/- 0.6 nmol GSH degraded/mg protein per min (mean +/- S.D.); cleavage activity by intact slices was 3.5 +/- 0.7 (P less than 0.001 relative to whole cortex homogenate) and in tubule fragments 9.4 +/- 0.8 (P less than 0.001). Homogenizing the tissue preparation increased cleavage rate in slices about 4-fold (12.4 +/- 2.9; P less than 0.005 relative to intact slice) but did not change the rate in tubule fragments (9.8 +/- 0.5). Maltose cleavage activity in whole cortex homogenate was 512 +/- 22 nmol glucose formed/mg protein per min, in slices 162 +/- 12, and in tubules 884 +/- 48. These findings imply that substrate in the incubation medium has a limited access to the luminal membrane of cortex slices but not of tubule fragments. They further imply that basolateral membrane is preferentially exposed in the slice preparation.     

Window panes of eternity. Health, disease, and inherited risk

Personal health reflects harmony between individual and experience; it is optimal homeostasis. Disease is an outcome of incongruity leading to dishomeostasis. Relative to earlier times, disease in modern society has higher "heritability" (in the broad meaning of the term). Inherited risks are facts compatible with anticipation and prevention of disease. This viewpoint has major implications for medical practice, deployment of health services, themes of research, and education of health care personnel and citizens.