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C McMartin

Publications and source records attributed to C McMartin.

At least 37 records · Page 2Linked to original sources
PubMed1980

Mechanisms of catabolism of corticotrophin-(1--24)-tetracosapeptide in the rat in vivo.

The fragmentation of corticotrophin-(1--24)-tetracosapeptide in vivo has been studied, using tritium-labelled hormone and chromatography, in the rat. After intravenous injection the levels of peptide in the circulation declined rapidly caused by its distribution to the tissues and from 2 min after injection a range of different cleavage products appeared. Many of the fragments in the circulation after 2 min have been identified and in this way cleavage has been shown to occur after residues 1, 2, 8, 15, 16, 17, 19, 20 and 21. It is believed that this is the result of aminopeptidase attack at the NH2 terminal, and of attack on the basic region of the molecule by trypsin-like endopeptidase followed by carboxypeptidase. The sulphoxide has been identified as a major metabolite in some experiments but the extent of its formation was very variable. Seventy per cent of the dose was distributed to the tissue beds by 1 min. Part of this was present, mainly as intact peptide, in liver and kidney but the greater proportion was found in muscle, skin and intestine where extensive degradation had already occurred. Further characterization of the fragments formed in the muscle provided good evidence that this tissue may have been the site of generation of many of the fragments which later appeared in the circulation.

Adrenocorticotropic Hormone↗
PubMed1979

Distribution and degradation of two tritium-labelled corticotrophin analogues in the rat.

The distribution and degradation of corticotrophin-(1--24)-tetracosapeptide specifically labelled with tritium at Tyr2, Phe7 or Tyr23 and [D-Ser1, Lys17, Lys18]-corticotrophin-(1--18)-octadecapeptide amide labelled at Tyr2 were studied at various times after intravenous injection into rats. By characterizing the radioactivity in plasma and various tissues, an overall picture of the metabolic handling of the two peptides emerged. The peptides left the circulation rapidly, entering mainly muscle and skin where they were extensively degraded. The D-Ser1-containing analogue was less rapidly degraded and intact peptide persisted in muscle and skin for up to 1 h. This peptide probably returned to the circulation giving rise to the sustained plasma levels observed after injection of the D-Ser1-substituted octadecapeptide but not after injection of the tetracosapeptide. Although initially the kidneys did not clear such large amounts of peptide as did muscle and skin they played an important role by continuously and, on the basis of existing evidence, irreversibly clearing the peptides and peptide fragments from the circulation and degrading them.

Adrenocorticotropic Hormone↗
PubMed1978

Early fate of somatostatin in the circulation of the rat after intravenous injection.

The products of somatostatin in the circulation have been investigated by high-pressure liquid chromatography. Plasma collected 1 min after intravenous injection of cyclic (oxidized) somatostatin showed a single ultraviolet absorbing peak. The total plasma content of this product was equivalent to 10--20% of the injected dose. Amino acid analysis showed that 80--90% of the material in the peak was [des-Ala1]-somatostatin and the remainder was unchanged peptide. [des-Ala1]-Somatostatin is rapidly formed in blood and plasma in vitro and according to other workers may be fully active. In contrast, 1 min after injection of linear (reduced) somatostatin, no products could be detected in the circulation. Incubations in vitro resulted in rapid conversion of the linear somatostatin to a product similar to the cyclic form. However, in vivo, very efficient clearance of linear somatostatin must occur even more rapidly than cyclization. In view of the very different clearance rates of the two forms of somatostatin, it is important to know whether endogenous somatostatin is released in the cyclic or the linear form. The absence of detectable concentrations of inactive peptide fragments in the circulation suggests that inactivation of somatostatin occurs in the tissues.

Amino Acids↗
PubMed1977

Use of octadecasilyl-silica for the extraction and purification of peptides in biological samples. Application to the identification of circulating metabolites of corticotropin-(1-24)-tetracosapeptide and somatostatin in vivo.

Peptides can be adsorbed on octadecasilyl-silica from large volumes of aqueous solution and eluted with aqueous solvent mixtures containing methanol or acetonitrile. These properties may be used for the extraction and purification of peptide fragments in plasma samples collected from rats. After intravenous injection of Synacthen [corticotropin-(1-24)-tetracosapeptide], it was shown that within 2 min the main circulating products were intact peptide and its sulphoxide. In addition, a number of fragments indicative of cleavage at the N- and C-termini were present. Most of the products formed from Synacthen have low biological activity. Somatostatin was rapidly cleaved in vivo and in vitro to a single product, which probably retains biological activity. The absence of other circulating products suggests that somatostatin is only inactivated once it leaves the circulation.

Adrenocorticotropic Hormone↗
PubMed1977

Differences between in-vitro and in-vivo potencies of corticotrophins: an interpretation in terms of metabolic stability.

Relative activities of a series of corticotrophin analogues have been measured by means of five different bioassays using the rat. Similarities in the relative potencies of various ACTH analogues determined using lipolysis or steroidogenesis in vivo and for the lipolytic and steroidogenic responses of fat pads and adrenal slices in vitro emerged and support the concept of a close structural relationship between the ACTH receptors in adipose and adrenal tissues in the rat. Potencies based on the steroidogenic response of isolated adrenal cells, adrenal slices or in-vivo experiments differed markedly from each other. Inactivation of peptides did not occur in the isolated cell assay, so it is likely that this assay estimates potency at the receptor level. A number of arguments suggest that the difference between the isolated cell assay and the other steroidogenic assays lies solely in the effects of peptide inactivation in the latter, and this allows the relative metabolic stabilities for the peptide analogues in these assays to be calculated. In this way it can be shown that: (1) Replacement of L-Ser by D-Ser in amino acid position 1 markedly increases the metabolic stability of the peptide and has only a slight effect on receptor properties. (2) Shortening at the NH2-terminus reduces the activity of peptides at the receptor level by several orders of magnitude, but increases their relative metabolic stability. (3) Introduction of amide groups at the CO2H-terminus markedly increases receptor potency of (1-16), (1-17) and (1-18) ACTH without affecting their metabolic stability in vivo. However, amidation of the CO2H-terminus does have a large effect on metabolic stability in the adrenal slice assay. (4) Replacement of Arg by Lys in positions 17 and 18 of (1-18) ACTH increases potency at the receptor level (adrenal cells) but has little effect on metabolic stability. The comparison of potencies obtained in the various assays, therefore, throws light on the significance of each assay. In addition, the effects of structural modification of analogues can be separately evaluated with respect to the metabolic stability of a peptide and its potency at the receptor level.

Adrenal Cortex Hormones↗
PubMed1977

Renal uptake and metabolism of adrenocorticotrophin analogues in the rat: an autoradiographic study.

Renal resorption of tritiated adrenocorticotrophin analogues was studied in the rat using light microscopic and quantitative electron microscopic autoradiography. The synthetic corticotrophins used were Synacthen (corticotrophin-(1-24)-tetracosapeptide) and C41795-Ba ([D-Ser1,Lys17,Lys18]-corticotrophin-(1-18)-octadecapeptide amide), the tetracosapeptide being tritiated in either the tyrosine residue of position 2 or 23 or the phenylalanine of position 7 and the octadecapeptide in the tyrosine of position 2. Inspection of autoradiographs showed that peptides injected intravenously were resorbed into proximal tubules by endocytosis to produce vesicles whose radiolabel later appeared in lysosomes, a route previously elucidated for other peptides and proteins. The use of two techniques for analysis of electron microscopic autoradiographs, however, suggested that apical tubules also acquire label and are in some way involved in the transfer of resorbed labelled material from endocytotic vesicles to lysosomes. In addition, the autoradiographic analyses revealed that the duration of lysosomal labelling depends upon the position of tritium in the chain. Thus, when the CO2H-terminus of Synacthen was labelled, silver grains were more transiently associated with lysosomes than was the case when the NH2-terminal or core regions were tritiated, indicating a greater resistance of these portions of the peptide to attack by intracellular peptidase. The label from the chemically protected C 41795-Ba was also less readily expelled from the lysosomes of the proximal tubules.

Adrenocorticotropic Hormone↗
PubMed1976

On the metabolism of two adrenocorticotrophin analogues.

The metabolism of Synacthen (corticotrophin-(1--24)-tetracosapeptide) and C-41795-Ba ([D-Ser1,Lys17,Lys18]-corticotrophin-(1--18)-octadecapeptide amide) have been compared in the rat following intravenous injection. Using Synacthen and C-41795-Ba labelled with tritium it was possible to follow the tissue distribution of the two peptides. The kidney was shown to concentrate intact peptide and fragments of both peptides. Autoradiography of perfused kidney sections demonstrated the uptake of radioactivity into the lysosomes of the kidney proximal tubule cells. Comparison of the distribution of radioactive products formed from the tetracosapeptide labelled in different positions indicates that, prior to renal uptake, metabolism is taking place at other sites in the body. It is suggested that rapid cleavage occurs extracellularly at or near the N- and C-termini. Then large fragments remaining in the circulation, together with any intact material, are filtered through the glomerulus and are taken up by endocytosis into the proximal tubule cells of the kidney. The synthetic N- and C-terminal protection of the octadecapeptide appears to inhibit the extracellular attack and so the concentration of intact peptide in the kidneys is initially higher than the tetracosapeptide. Although concentrations of radioactivity in other tissues are low in comparison with the kidneys, the quantities are quite high when the weight of the tissues are considered. Chromatographic analysis of this radioactivity reveals that the octadecapeptide gives rise to much higher tissue levels of intact peptide and we believe that this acts as a depot and gives rise to the sustained blood concentrations and prolonged biological effects observed with this peptide.

Adrenocorticotropic Hormone↗
PubMed1975

An investigation of the involvement of adenosine 3':5'-cyclic monophosphate in steroidogenesis by using isolated adrenal cell column perfusion.

The involvement of cyclic AMP in corticosteroidogenesis was investigated by using isolated adrenal cell column perfusion. Steroids were produced in response to 0.5, 1.0 and 5.0 mg of cyclic AMP/ml. Analysis of the shape of the response curves indicated an inverse relationship between rate of onset of steroid production and dose. A further increase in steroid production during the washout period after the 5 mg/ml dose was considered to indicate an intracellular inhibitory effect of cyclic AMP. Release of cyclic AMP into the perfusate only occurred in response to supramaximal steroidogenic doses of ACTH (adrenocorticotrophin). A connexion between dose and response was demonstrated over a narrow concentration range. Variation in the time-lag before cyclic AMP production and in the duration of the response was marked; further, no reproducible ratio of steroid output to cyclic AMP output was shown at any level of stimulation. These results are discussed together with those of other recent investigations. It is considered that these findings do not support an obligatory role for cyclic AMP as mediator of ACTH action in the adrenal.

Adrenal Cortex Hormones↗
PubMed1975

Levels of corticotrophin analogues in the blood after infusion into rats.

An isolated rat adrenal cell bioassay was used to measure blood concentrations in rats after infusion of synthetic human ACTH, corticotrophin-(1-24)-tetracosapeptide or [D-Ser1, Lys17, Lys18]corticotrophin-(1-18)-octadecapeptide amide. Lower blood levels were found with the 1-24 peptide than with human ACTH and the highest levels were found with the 1-18 peptide. These results suggest that the 1-24 peptide which is almost equipotent with natural ACTH in vivo may be more potent at the receptor and corroborate findings to this effect obtained with isolated adrenal cells. The high potency and prolonged action of the 1-18 analogue in vivo are also explained by these results. Low arterial blood concentrations of the 1-24 peptide and human ACTH were found during infusion, suggesting that substantial inactivation must be occurring in a single passage through the lungs. The effects of renal ligature on blood concentrations indicated that the kidney is involved in handling the 1-18 peptide and that human ACTH is also cleared by this organ. After infusion the fall in blood concentrations was biphasic. It is suggested that the rapid phase is due to clearance of peptides in the circulation which results in a fall to lower blood concentrations which are sustained by slow release of peptide from binding sites which act as a depot.

Adrenocorticotropic Hormone↗
PubMed1974

Metabolism of two adrenocorticotrophin analogues in the intestine of the rat.

1. At 30min after oral administration unchanged Synacthen [corticotrophin-(1-24)-tetracosapeptide] was found in the stomach but could not be detected in the lumen of the small intestine of the rat. 2. Synacthen and 41795-Ba {[d-Ser(1), Lys(17), Lys(18)]corticotrophin-(1-18)-octadecapeptide amide} were rapidly metabolized in vitro by both intestinal juice and everted pieces of small intestine. Peptide products were not found either in the intestinal tissue or in the fluid bathing the serosal tissue. 3. Glucose but not O(2) was necessary for the breakdown of the two adrenocorticotrophin analogues by everted tissue. 4. When the products obtained after partial digestion were chromatographically separated and identified, a pattern of breakdown emerged. The N-terminus of Synacthen and the Phe(7)-Arg(8) bond in both analogues were particularly labile. The d-serine N-terminal residue of 41795-Ba conferred a marked protection to aminopeptidase action. 5. The relative susceptibilities of peptide bonds would have been difficult to predict on the basis of existing knowledge of the properties of enzymes of the small intestine.

Adrenocorticotropic Hormone↗
PubMed1974

Fate of corticotrophins in an isolated adrenal-cell bioassay and decrease of peptide breakdown by cell purification.

1. The fate of corticotrophins in a trypsin-dispersed rat adrenal-cell assay system was investigated with a view to establishing whether differences in the rate of inactivation might contribute to potency differences observed between analogues. 2. Corticotrophin-(1-24)-tetracosapeptide and to a lesser extent synthetic 1-39 corticotrophins were found to be inactivated during incubation with cell suspension. 3. Peptide fragments were isolated by using [[(3)H(2)]Tyr(23)]corticotrophin-(1-24)- tetracosapeptide as a marker. The fragments indicate a peptidase with a predominantly tryptic specificity. 4. The peptidase is present in the extracellular fluid and is released from cells when they are damaged. 5. Cells were fractionated on an albumin gradient. Cells from the zona fasciculata and the zona intermedia or reticularis were present in fractions which produced fluorogenic steroids in response to corticotrophin. 6. Purification of the cells by centrifugation through albumin decreased degradation by peptidases, so that if the assay is carried out with a dilute suspension of purified cells peptide breakdown should not affect the observed potencies of adrenocorticotrophin analogues. 7. No binding of [[(3)H(2)]Tyr(23)]corticotrophin-(1-24)- tetracosapeptide to cells could be detected at low concentrations of the peptide. This indicated that less than 120 receptors/cell are occupied during stimulation by a dose that would elicit approx. 80% of the maximal response.

Adrenal Glands↗