Selenocysteine confers the biochemical properties characteristic of the type I iodothyronine deiodinase.

The conversion of thyroxine to 3,5,3'-triiodothyronine (T3) is the first step in thyroid hormone action, and the Type I iodothyronine deiodinase supplies most of this extrathyroidal T3 in the rat. We found that the cDNA coding for this enzyme contains an in-frame UGA encoding the rare amino acid selenocysteine. Using site-directed mutagenesis, we have converted selenocysteine to cysteine and expressed the wild-type and cysteine mutant enzymes in JEG-3 cells by transient transfection. The kinetic properties of the transiently expressed wild-type enzyme are nearly identical to those reported for rat liver Type I deiodinase. Substitution of sulfur for selenium causes a 10-fold increase in the Km of the enzyme for the favored substrate 3,3',5'-triiodothyronine (rT3), a 100-fold decrease in the sensitivity of rT3 deiodination to competitive inhibition by gold and a 300-fold increase in the apparent Ki for uncompetitive inhibition by 6-n-propylthiouracil. These results demonstrate that selenium is responsible for the biochemical properties which characterize Type I iodothyronine monodeiodination.     

Angiotensin III and IV activation of the brain AT1 receptor subtype in cardiovascular function

The present investigation determined that native angiotensins II and III (ANG II and III) were equipotent as pressor agents when ICV infused in alert rats, whereas native angiotensin IV (ANG IV) was less potent. An analogue of each of these angiotensins was prepared with a hydroxyethylamine (HEA) amide bond replacement at the N-terminus, yielding additional resistance to degradation. These three angiotensin analogues, HEA-ANG II, HEA-ANG III, and HEA-ANG IV, were equivalent with respect to maximum elevation in pressor responses when ICV infused; and each evidenced significantly extended durations of effect compared with their respective native angiotensin. Comparing analogues, HEA-ANG II had a significantly longer effect compared with HEA-ANG III, and HEA-ANG IV, whereas the latter were equivalent. Pretreatment with the AT₁ receptor subtype antagonist, Losartan (DuP753), blocked subsequent pressor responses to each of these analogues, suggesting that these responses were mediated by the AT₁ receptor subtype. Pretreatment with the specific AT₄ receptor subtype antagonist, Divalinal (HED 1291), failed to influence pressor responses induced by the subsequent infusion of these analogues. These results suggest an important role for Ang III, and perhaps ANG IV, in brain angiotensin pressor responses mediated by the AT₁ receptor subtype.     

Species-dependent features of Dogiel type II neurones in the mammalian enteric nervous system.

Although autonomic gastrointestinal reflex movements, which occur in all mammalian species, have been described almost a century ago, little was known on the mechanisms underlying this behaviour. Recently, however, intrinsic primary afferent neurones, functioning as the first relay in the reflex arches embedded in the intestinal wall, have been identified in the guinea pig ileum. In guinea pig, such neurones display a Dogiel type II morphology and behave electrophysiologically as slow AHP neurones. In other gastrointestinal regions, in both guinea pig and rat, Dogiel type II cells are also encountered, but the strong correlation with slow AHP neuronal features seems less strict. In large mammals, a correlation of the cellular morphology with intracellular el ectrophysiological recordings has only been obtained in the pig small intestine. Surprisingly, in these experiments aberrant electrophysiological behaviour of Dogiel type II neurones is even more striking since the majority of these cells display electrophysiological features considered typical of S neurones. Furthermore, in those rare cases in which a slow afterhyperpolarization (AHP) could be recorded in porcine Dogiel type II cells, its amplitudes were negligible. This has led us to the conclusion that the differences in electrophysiological behaviour of neurones with comparable morphology in different species are most probably due to the modulating influence of the neurotransmitter substances present. This seems to be the most likely hypothesis in view of the considerable differences in neurotransmitter content of neurones with comparable functions throughout the species.     

The crystal structure of cystathionine gamma-synthase from Nicotiana tabacum reveals its substrate and reaction specificity.

Cystathionine gamma-synthase catalyses the committed step of de novo methionine biosynthesis in micro-organisms and plants, making the enzyme an attractive target for the design of new antibiotics and herbicides. The crystal structure of cystathionine gamma-synthase from Nicotiana tabacum has been solved by Patterson search techniques using the structure of Escherichia coli cystathionine gamma-synthase. The model was refined at 2.9 A resolution to a crystallographic R -factor of 20.1 % (Rfree25.0 %). The physiological substrates of the enzyme, L-homoserine phosphate and L-cysteine, were modelled into the unliganded structure. These complexes support the proposed ping-pong mechanism for catalysis and illustrate the dissimilar substrate specificities of bacterial and plant cystathionine gamma-synthases on a molecular level. The main difference arises from the binding modes of the distal substrate groups (O -acetyl/succinyl versusO -phosphate). Central in fixing the distal phosphate of the plant CGS substrate is an exposed lysine residue that is strictly conserved in plant cystathionine gamma-synthases whereas bacterial enzymes carry a glycine residue at this position. General insight regarding the reaction specificity of transsulphuration enzymes is gained by the comparison to cystathionine beta-lyase from E. coli, indicating the mechanistic importance of a second substrate binding site for L-cysteine which leads to different chemical reaction types.     

The J-related segment of tim44 is essential for cell viability: a mutant Tim44 remains in the mitochondrial import site, but inefficiently recruits mtHsp70 and impairs protein translocation.

Tim44 is a protein of the mitochondrial inner membrane and serves as an adaptor protein for mtHsp70 that drives the import of preproteins in an ATP-dependent manner. In this study we have modified the interaction of Tim44 with mtHsp70 and characterized the consequences for protein translocation. By deletion of an 18-residue segment of Tim44 with limited similarity to J-proteins, the binding of Tim44 to mtHsp70 was weakened. We found that in the yeast Saccharomyces cerevisiae the deletion of this segment is lethal. To investigate the role of the 18-residue segment, we expressed Tim44Delta18 in addition to the endogenous wild-type Tim44. Tim44Delta18 is correctly targeted to mitochondria and assembles in the inner membrane import site. The coexpression of Tim44Delta18 together with wild-type Tim44, however, does not stimulate protein import, but reduces its efficiency. In particular, the promotion of unfolding of preproteins during translocation is inhibited. mtHsp70 is still able to bind to Tim44Delta18 in an ATP-regulated manner, but the efficiency of interaction is reduced. These results suggest that the J-related segment of Tim44 is needed for productive interaction with mtHsp70. The efficient cooperation of mtHsp70 with Tim44 facilitates the translocation of loosely folded preproteins and plays a crucial role in the import of preproteins which contain a tightly folded domain.