The primary structure of ratfish insulin reveals an unusual mode of proinsulin processing

Elephantfish proinsulin possesses a monobasic processing site

Total pancreatic RNA from the holocephalan species Callorhyncus milii (elephantfish) was used to make cDNA as a template for the polymerase chain reaction. Three redundant primers based on the known amino acid sequence of elephantfish insulin were used to amplify a fragment of proinsulin comprising truncated B-chain, complete C-peptide, and complete A-chain. Whereas the C-peptide/A-chain junction contained the expected dibasic cleavage site (-Lys-Arg-), the B-chain/C-peptide junction was found to contain only a single Arg, the first such site to be unequivocally associated with the proteolytic processing of a proinsulin to insulin. Examination of the flanking sequences around this site shows that a typical endocrine/neuroendocrine PC3 conversion enzyme should still be able to cleave, as the general requirements for precursor processing at a monobasic site are satisfied, notably a basic residue (Lys) at the -4 position. An acidic residue (in this case Asp) at the +1 position, which is seen in all known proinsulins, is maintained. The corresponding genomic DNA fragment of elephantfish proinsulin was also amplified by PCR, revealing a 402-bp intron at the conserved IVS-2 position within the C7 codon.

Primary structure of gonadotropin-releasing hormone from the brain of a holocephalan (ratfish: Hydrolagus colliei)

Gonadotropin-releasing hormone (GnRH) in the ratfish brain has been isolated and purified using reverse-phase high performance liquid chromatography. Amino acid composition and sequence analysis indicate that the primary structure is pGlu-His-Trp-Ser-His-Gly-Trp-Tyr-Pro-Gly-NH2. The presence of the amino terminal pyroglutamic acid has been confirmed by degradation studies with pyroglutamyl aminopeptidase. The amidated carboxy terminus and molecular weight were confirmed using mass spectrometry. Moreover, sequence comparison and coelution studies with one of the synthetic forms of GnRH (chicken GnRH II) indicate that the ratfish and chicken GnRH II molecules are identical. This represents the first sequence data of a GnRH molecule from a cartilaginous fish (class: Chondrichthyes). It is argued that the ratfish GnRH molecule has been retained for over 400 million years of evolution and is expressed in most vertebrate classes.

Isolation and structural characterization of insulin and glucagon from the holocephalan species Callorhynchus milii (elephantfish)

Both insulin and glucagon from the pancreas of the holocephalan cartilaginous fish Callorhynchus milii (elephantfish) have been isolated and purified. Two reverse-phase h.p.l.c. steps enabled recovery of sufficient material for gas-phase sequencing of the intact chains as well as peptide digestion products. The elephantfish insulin sequence shows 14 differences from pig insulin, including two unusual substitutions, Val-A14 and Gln-B30, though none of these is thought likely to influence receptor binding significantly. The insulin B-chain contains 31 residues, one more than mammalian insulins, but markedly less than that of the closely related ratfish with which it otherwise exhibits high sequence similarity. Elephantfish and pig glucagons differ at only four positions, but there are six changes from the ratfish glucagon-36 (normal glucagon contains 29 residues) sequence. It is apparent that different prohormone proteolytic processing mechanisms operate in the two holocephalan species.

Purification of a marsupial insulin: amino-acid sequence of insulin from the eastern grey kangaroo Macropus giganteus

Insulin has been purified from kangaroo pancreas by acidic ethanol extraction, diethyl ether precipitation and gel filtration. The amino-acid sequence of this, the first marsupial insulin to be studied, is reported. It differs from human insulin by only four amino-acid substitutions, all in regions of the molecule previously known to be variable. However, it should be noted that one of these, asparagine for threonine at A8, has not been reported before. Computer comparisons of all 43 insulin sequences reported to date with kangaroo insulin show it to be most closely related to a group of mammalian insulins (dog, pig, cow, human) known to be of high biological potency. The measurement of blood glucose lowering in the rabbit by kangaroo insulin is consistent with this conclusion. Comparisons of amino-acid sequences of other proteins with their kangaroo counterparts show a greater difference, in line with the time of divergence of marsupials. The limited differences observed in insulin and cytochrome c suggest that their structures need to be closely conserved in order to maintain function.

Isolation and primary structure of the C-peptide of proinsulin from the European eel (Anguilla anguilla)

1. The primary structure of the C-peptide of proinsulin from the European eel has been established as: DVEPLLGFLSPKSGQENEVDDFPYKGQGEL. The peptide was isolated from the extract of eel pancreas in a yield that was approximately equimolar with insulin. A comparison with the predicted structures of C-peptides from other teleost fishes has identified a domain in the central region of the peptide that has been more highly conserved than the rest of the molecule.

Multiple molecular forms of insulin and glucagon-like peptide from the Pacific ratfish (Hydrolagus colliei)

The primary structure of an insulin isolated from the pancreas of the holocephalan fish, Hydrolagus colliei (Pacific ratfish), has been established as A-chain: GIVEQCCHNTCSLANLEGYCN B-chain: VPTQRLCGSHLVDALYFVCGERGFFYSPKPIRELEPLL. Three further molecular forms of insulin were also isolated and shown to have the same A-chain but truncated B-chains of 31-, 36-, and 37-amino acid residues. It is proposed that all four insulins arise from a single proinsulin by proteolytic cleavages at different sites within the C-peptide region. The insulin with 38 amino acids in the B-chain was equipotent with human insulin in inhibiting the binding of radiolabelled human insulin to rat fat cells but the maximum effect of ratfish insulin upon the transport of 3-O-methylglucose into the cells was only 65% of the maximum effect of human insulin. Two molecular forms of glucagon-like peptide were isolated from the ratfish pancreas. The primary structure of the more abundant peptide was established as HADGIYTSDVASLTDYLKSKRFVESLSNYNRKQND. The primary structure of the second peptide was the same except that it was extended from the C-terminus by the sequence RRM. It is probable, therefore, that both glucagon-like peptides also arise from a single proglucagon by different pathways of post-translational processing.

Isolation and structural characterization of insulin from the holocephalan fish, Chimaera monstrosa (rabbit fish)

Insulin has been isolated from the pancreas of the holocephalan fish, Chimaera monstrosa (rabbit fish), and characterized by automated Edman degradation and fast atom bombardment mass spectrometry. The primary structure of rabbit fish insulin was identical to that of insulin from the holocephalan fish, Hydrolagus colliei (Pacific ratfish), and contained 21 residues in the A-chain and 38 residues in the B-chain. The amino acid compositions of both rabbit fish and ratfish insulins demonstrated a value consistently lower than that expected for the leucine content of the peptides. It is suggested, therefore, that the insulins were probably isolated as a mixture of the intact peptides and components lacking the C-terminal leucine residue in the B-chain.

Isolation and structure of lamprey (Petromyzon marinus) insulin

Insulin has been purified to homogeneity from the caudal and cranial pancreas of the adult sea lamprey (Petromyzon marinus). The final yield was 18.6 nmol/g (127.8 micrograms/g). The structures of both A- and B-chains have been determined using amino acid analyses, gas-phase sequence analyses, and proteolytic mapping by fast atom bombardment-mass spectrometry. The sequence of the A-chain was found to be GIVEQCCHRKCSIYDMENYCN. The sequence of the B-chain, extended at the amino terminus, was determined to be SALT-GAGGTHLCGSHLVEALYVVCGDRGFFYTPSKT. Lamprey insulin retains the common features of vertebrate insulins. Sea lamprey insulin has no more homology to hagfish (Myxine glutinosa) insulin than it has to the teleost fish or to mammalian insulins.

A glucagon-like peptide, structurally related to mammalian oxyntomodulin, from the pancreas of a holocephalan fish, Hydrolagus colliei

The pancreatic islets of the holocephalan fishes contain, in addition to A-, B- and D-cells, X-cells, which are immunoreactive towards antisera directed against the N-terminal region of glucagon but not towards antisera directed against the C-terminal region. A 36-amino-acid-residue peptide was isolated from the pancreas of a holocephalan fish, the Pacific ratfish (Hydrolagus colliei), that shows homology (69%) to mammalian glucagon in its N-terminal region and is reactive towards an N-terminally directed antiserum. Reactivity towards C-terminally directed antisera is prevented by the presence of a 7-residue C-terminal extension to the glucagon sequence that shows limited homology to the C-terminal region of glucagon-37 (oxyntomodulin). It is proposed that this peptide represents a major storage product of the islet X-cell.

Primary structure of insulin and glucagon from the flounder (Platichthys flesus)

Insulin and glucagon have been isolated from the Brockmann bodies of the flounder, a teleostean fish, and their primary structures established by automated Edman degradation. The A-chain of flounder insulin shows strong homology to the A-chains from the coho salmon (Oncorhynchus kisutch; 100%) and the anglerfish (Lophius americanus; 95%) but homologies in the B-chain region are weaker (salmon 79%, anglerfish 83%). Flounder insulin B-chain contains the novel sequence Val-Val-Pro-Pro at the NH2 terminus and the highly conserved seryl residue at position 10 (B 9 in mammals) is replaced by an alanyl residue. Flounder glucagon is identical to anglerfish glucagon II but shows four amino acid substitutions compared with salmon glucagon.