Displacement of rhodopsin by GDP from three-loop interaction with transducin depends critically on the diphosphate beta-position

We have studied the effect of GDP and its analog guanyl-5'-yl thiophosphate (GDP beta S) on the interaction between rhodopsin and transducin (Gt). Stabilization of the light-induced active intermediate, metarhodopsin II (MII), by bound Gt (extra-MII effect) monitored the catalytic interaction between the proteins. Extra-MII can be completely abolished by GDP, with a half-suppression at 10 microM under the conditions (4 degrees C, pH 8, 7.5 nM photoactivated rhodopsin). The effect of GDP did not depend on divalent cations, in contrast to GTP-induced dissociation of the complex. The GDP analog GDP beta S did not affect extra-MII although it binds to the MII-Gt complex with only three times lower affinity (reversal of the GDP effect by GDP beta S). However, GDP beta S enhanced considerably the efficiency of synthetic rhodopsin peptide competition against the formation of extra-MII. GDP and GDP beta S slow the Gt activation rate (monitored by kinetic light scattering), with the same relative efficiencies. We therefore assume that GDP, GDP beta S, and GTP bind at the same site. We discuss a generalized induced fit mechanism, where MII induces opening of the Gt nucleotide site and release of GDP which in turn is obligatory to establish the MII-stabilizing rhodopsin-Gt three-loop interaction (K?nig, B., Arendt, A., McDowell, J.H., Kahlert, M., Hargrave, P.A., and Hofmann, K.P. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 6878-6882). The GDP beta S/GDP difference is discussed in terms of bound GDP disturbing the interaction with two and GDP beta S with only one of the rhodopsin binding sites. Mechanistically, our results indicate a critical role of the beta-phosphate interaction with the nucleotide binding site in the GDP-induced transformation of Gt.     

Cellular lung dosimetry for inhaled radon decay products as a base for radiation-induced lung cancer risk assessment

Summary Lung cancer induction is commonly regarded as the most important somatic risk arising from the inhalation of radon and its decay products. Relating carcinogenesis to radiation exposure needs a detailed knowledge of the cellular dose distribution in the human respiratory tract. Different dosimetric models have been developed for the determination of cellular doses, particularly for the basal cells of the bronchial epithelium which are considered as the critical cells for lung cancer induction. Part I of the paper describes the influence of various environmental as well as anatomical and physiological factors on the resulting dose. Significant inter- as well as intra-subject variabilities of structural components of the human lung, respiration characteristics and clearance mechanisms demonstrate the necessity of applying stochastic models in lung dosimetry.     

The steroidogenic activity of biotinylcorticotropin-avidin complexes.

The steroidogenic activity of complexes of [biocytin²⁵]-corticotropin_1–25 amide (biotinylcorticotropin) with avidin (1:1), (4:1) and (1:10) was compared to that of biotinylcorticotropin using isolated rat adrenal cells. Parallel log-dose responses and maximal stimulation were observed with all these materials. The 1:1 complex is approximately 25% as active as biotinylcorticotropin (ED₅₀22.5 and 5.6 $\text{nM}$ respectively). The 4:1 complex is more active than the 1:1 complex (ED₅₀9.0 $\text{nM}$). The presence of an excess of avidin (1:10 complex) does not interfere with the ability of biotinylcorticotropin to stimulate steroidogenesis (ED₅₀18.0 $\text{nM}$). It is concluded that biotinylcorticotropin attached to avidin binds specifically to receptors on the rat adrenal cell and elicits its biological response. These results indicate that biotinylcorticotropin can be noninvasively labeled with ¹²⁵I-avidin.     

Degradation and dehalogenation of monochlorophenols by the phenol-assimilating yeast Candida maltosa

The phenol-assimilating yeast Candida maltosa is able to degrade monochlorophenols but cannot grow on these substrates. 3- and 4-chlorophenol were broken down very rapidly by phenol-grown cells under the formation of 4-chlorocatechol, 5-chloropyrogallol and 4-carboxymethylenebut-2-en-4-olide with concomitant release of chloride.2-Chlorophenol was partially converted into cis,cis-2-chloromuconic acid via 3-chlorocatechol which was also obtained from 3-chlorophenol in low amounts. No further metabolites containing chloride were found.The dehalogenation step in the chlorophenol degradation is the cycloisomerization of the cis,cis-chloromuconic acid to 4-carboxymethylenebut-2-en-4-olide in the ortho fission pathway.Dedicated to prof.Dr. E. Bayer, Tübingen, on the occasion of his 65th birthday.     

Weitere Untersuchungen zur Phylogenetik der Pseudocholinesterasen

Ohne ZusammenfassungErste Mitteilung: H. W. Goedde u. W. Fuss, Humangenetik 1, 126 (1964).     

Morphological characteristics of the masseter muscle of 22 ruminant species

The bilateral masseter muscle was dissected from formalin preserved heads of 41 ruminants belonging to 22 species and three feeding types. Topographic relations of the masseter portions were reexamined in relation to mandibular shape. In contrast to earlier observations, masseter weight is significantly correlated with body weight irrespective of body size and feeding type, amounting in all ruminants to approximately 0.20% of body weight. Morphophysiological differences in masseter attachment and leverage are due to the different arrangements of masseter muscle tissue.     

Mechanism and pathway of penicillopepsin-catalyzed transpeptidation and evidence for noncovalent trapping of amino acid and peptide intermediates

Penicillopepsin acting on Nph-Ala2-amide (where Nph = p-nitrophenylalanyl) catalyzes a transpeptidation reaction which leads to the formation of Nph2-Ala2-amide, which arises from condensation of the substrate with enzyme-bound Nph, as the first product released from the enzyme. This is followed by a stage during which Nph3 and Ala2-amide are the major products. A small amount of Nph4 is also formed during this time. Nph and Nph2, formed during the reactions, are tightly, but probably not covalently, bound to the enzyme. They appear as free products only as a result of the cleavage of Nph3 and Nph4 and after most of the substrate Nph-Ala2-amide has been used up. They act as acceptors for the substrate and for Nph2-Ala2-amide. Nph3-Ala2-amide, formed by condensation of Nph-Ala2-amide or of Nph2-Ala2-amide with enzyme-bound Nph2 or Nph, respectively, is also released but is cleaved rapidly to give Nph3 and Ala2-amide. Incorporation of 18O from [18O]water into the carbonyl oxygens of the products is extensive and shows that release of the intermediates is slower than peptide bond cleavage and peptide bond formation. Hence the rate-limiting step in these reactions is product release. No 18O is incorporated into the initial substrate. We propose that Nph and Nph2 as intermediates are held in the active site by hydrogen bonds and by two strong electrostatic interactions.