Insulin and glucagon, two of the most studied pancreatic hormones bind to specific membrane receptors to exert their biological actions Insulin-like growth factors IGF-I and IGF-II are structurally related to insulin, although they are expressed ubiquitously The biological functions of the IGFs are mediated by different transmembrane receptors, which includes the insulin, IGF-I and IGF-II receptors The interaction of insulin, insulin related peptides and glucagon with the corresponding receptors has been studied extensively in mammals and continues to be so At the same time, research on ectothermic animals has made enormous progress in the recent years This paper summarizes current knowledge on insulin, IGF-I and glucagon receptors, from a comparative point of view with special attention to non-mammalian vertebrates The review covers adult and mostly typical target tissues, and with very few exceptions, developmental aspects are not considered Binding characteristics, tissue distribution and structure of insulin and IGF-I receptors will be considered first, because both ligands and receptors are structurally related and have overlapping functions These sections will be followed by similar distribution of information on glucagon receptors Readers interested in either structure or functions of insulin, IGFs and glucagon in nonmammalian vertebrates are referred to other reviews (Mommsen TP, Plisetskaya EM Insulin in fishes and agnathans: history, structure and metabolic regulation Rev Aquat Sci 1991;4:225-259; Mommsen TP, Plisetskaya EM Metabolic and endocrine functions of glucagon-like peptides: evolutionary and biochemical perspectives Fish Physiol Biochem 1993;11:429-438; Duguay SJ, Mommsen TP Molecular aspects of pancreatic peptides In: Sherwood NM, Hew CL, editors, Fish Physiology vol 13 1994:225-271; Plisetskaya EM, Mommsen TP Glucagon and glucagon-like peptides in fishes Int Rev Citol 1996;168:187-257)

Insulin, insulin-like growth factor-I (IGF-I) and glucagon: the evolution of their receptors.

A thermoacidophilic, obligately chemolithoautotrophic, aerobic, hydrogen-oxidizing bacterium, strain 3H-1T(T = type strain), was isolated from a solfataric field in Tsumagoi, Japan. This strain is a gram-negative, motile, non-spore-forming rod-shaped organism that requires elemental sulfur for growth by hydrogen oxidation. Type b, c, and o cytochromes are present. Carbon dioxide may be fixed via the reductive tricarboxylic acid cycle. The optimum temperature for growth is 65°C. The optimum pH for growth is 3.0 to 4.0. The guanine-plus-cytosine content of DNA is 35.0 mol%. A straight-chain saturated C18:0acid and straight-chain unsaturated C18:1and C20:1acids are the major components of the cellular fatty acids. 2-Methylthio-3-VI, VII-tetrahydromultiprenyl7-1,4-naphthoquinone (methionaquinone) is the major isoprenoid quinone. This strain is considered a member of a new species of the genus Hydrogenobacter, a genus of obligately chemolithoautotrophic, aerobic, hydrogen-oxidizing bacteria. The name Hydrogenobacter acidophilus sp. nov. is proposed for the organism. The type strain of this species is strain 3H-1 (= JCM 8795).

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Hydrogenobacter acidophilus sp. nov., a Thermoacidophilic, Aerobic, Hydrogen-Oxidizing Bacterium Requiring Elemental Sulfur for Growth

The complete amino acid sequence of cytochrome c-552 from an extremely thermophilic hydrogen bacterium, Hydrogenobacter thermophilus TK-6 (IAM 12695), was determined. It is a single polypeptide chain of 80 residues, and its molecular weight, including heme, was calculated to be 7,599. The sequence of cytochrome c-552 from H. thermophilus TK-6 closely resembles that of cytochromes c-551 from Pseudomonas species. Moreover, the tertiary structure of Hydrogenobacter cytochrome c-552 is suggested to be similar to that of cytochrome c-551 from Pseudomonas aeruginosa. The sequence similarity between Hydrogenobacter cytochrome c-552 and that of other bacteria physiologically related to H. thermophilus is not high.

Amino acid sequence of cytochrome c-552 from a thermophilic hydrogen-oxidizing bacterium, Hydrogenobacter thermophilus.

Geothermal fluids and gases often contain significant concentrations of molecular hydrogen (≥1% v/v). This suggests that hot springs and other geothermal manifestations might harbor thermophilic, aerobic, hydrogen-oxidizing (Knallgas) bacteria. It is therefore surprising how few attempts were made before 1980 to isolate thermophilic hydrogen bacteria from these environments (Goto et al., 1977), although they were found in cold nongeothermal habitats (McGee et al., 1967; Emnova and Romanova, 1977; Aragno, 1978; Schenk and Aragno, 1979).

Thermophilic, Aerobic, Hydrogen-Oxidizing (Knallgas) Bacteria

An obligately chemolithoautotrophic and aerobic hydrogen-oxidizing bacterium was isolated from seawater of the Shonan Coast, Kanagawa Pref., Japan. The isolate was a Gram-negative, comma-shaped rod cell measuring 0.2 to 0.5 by 1 to 2 μm. The cells occurred singly and were motile by a polar flagellum. The deoxyribonucleic acid base composition of the isolate was 44.1 mol% guanine plus cytosine. The optimal temperature for autotrophic growth on H2−O2−CO2 was around 37°C, and no growth was observed at 5° C or 45° C. The optimal pH for growth was around 6.5. NaCl was required for growth with an optimum of 0.5 M. Elemental sulfur, thiosulfate or tetrathionate was utilized, as well as molecular hydrogen, as the sole energy source. No heterotrophic growth was observed on organic media tested. To our knowledge, this is the first report of the isolation of a marine, aerobic hydrogen-oxidizing bacterium, and of an obligately chemolithoautotrophic, mesophilic and aerobic hydrogen-oxidizing bacterium.

Isolation of an obligately chemolithoautotrophic, halophilic and aerobic hydrogen-oxidizing bacterium from marine environment

The cells of Hydrogenobacter thermophilus strain TK-6 contain only a membrane-bound hydrogenase. The membrane-bound hydrogenase was partially purified from the cells of H. thermophilus after solubilization by alkaline treatment. The hydrogenase activity was increased up to 10-fold by the addition of anionic detergents. The hydrogenase reacted with cytochrome c552 obtained from the same strain in the presence of an anionic detergent. Our experiments show cytochrome c552 is a natural electron acceptor for the membrane-bound hydrogenase, and an electron transport system in H. thermophilus strain TK-6 is proposed.

The Membrane-bound Hydrogenase Reduces Cytochrome c552 in Hydrogenobacter thermophilus strain TK-6

1. We have characterized the insulin receptor in primary cultured quail oviduct cells and examined the hormonal regulation of its level. 2. We have also shown the recycling pathway of insulin receptors in the cultured cells using specific inhibitors (tunicamycin, chloroquine, monensin, and brefeldin A). 3. Our data suggest that glucocorticoids play important physiological roles in egg-white protein synthesis through increasing the number of insulin receptors and insulin through enhancing the transport of amino acids.

Characterization of insulin receptors in primary cultures of quail (Coturnix coturnix japonica) oviduct cells. The level of insulin receptor is regulated by steroid and peptide hormones.

Shigehiko Itoh

Papers

The Membrane-bound Hydrogenase Reduces Cytochrome c552 in Hydrogenobacter thermophilus strain TK-6

Characterization of insulin receptors in primary cultures of quail (Coturnix coturnix japonica) oviduct cells. The level of insulin receptor is regulated by steroid and peptide hormones.