Isolation of alpha-melanotropin and N, O-diacetylserine1-alpha-melanotropin from porcine pituitary extracts

The posttranslational modification of beta-endorphin in the intermediate pituitary of the toad, Bufo marinus, includes processing at a monobasic cleavage site

Fractionation of an acid extract of 15 B. marinus intermediate pituitaries by a combination of gel filtration chromatography and cation exchange chromatography revealed one major and five minor forms of beta-endorphin in this tissue. Based on reversed-phase HPLC and immunological properties, as well as amino acid composition and primary sequence analysis, it was deduced that the sequence of the major form of B. marinus beta-endorphin is N-acetyl-YGGFMTPE. Overall, the steady-state analyses of the minor forms of beta-endorphin indicated that the posttranslational processing of beta-endorphin in the toad intermediate pituitary includes endoproteolytic cleavage at both paired basic and monobasic cleavage sites.

Differential N-acetylation of alpha-MSH and beta-endorphin in the intermediate pituitary of the turtle, Pseudemys scripta

Steady-state analyses of the intermediate pituitary of the turtle, Pseudemys scripta, indicated that alpha-MSH-sized immunoreactive forms and beta-endorphin-sized immunoreactive forms are major end products of melanotropic cells. Three forms of alpha-MSH-related immunoreactivity were detected. The two major forms had the same reversed-phase HPLC properties as synthetic N,O-diacetyl-ACTH(1-13)-NH2 and N-acetyl-ACTH(1-13)-NH2. These forms accounted for 97% of the total alpha-MSH-related immunoreactivity detected. A minor peak of ACTH(1-13)-NH2 was also detected. Multiple forms of beta-endorphin-related immunoreactivity were detected, which varied in net positive charge (+1 to +5), apparent molecular weight (2.4 to 3.5 kDa), and degree of N-terminal acetylation. Although N-acetylated forms of beta-endorphin were detected in the turtle intermediate pituitary, the major forms of turtle beta-endorphin were nonacetylated. These features of the turtle intermediate pituitary POMC-specific N-acetylation mechanism are similar to, yet distinct from, the POMC N-acetylation mechanisms observed for mammals. These data suggest that POMC-specific N-acetylation mechanisms were present in reptiles prior to the divergence of the anapsid and synapsid lines.

Immunological evidence for multiple forms of alpha-melanotropin (alpha-MSH) in the pars intermedia of the Australian lungfish, Neoceratodus forsteri

Acid extracts of individual pars intermedia from the Australian lungfish, Neoceratodus forsteri, were fractionated by gel filtration chromatography and analyzed for alpha-melanotropin (alpha-MSH) immunoreactivity. In these studies a C-terminal-specific alpha-MSH radioimmunoassay (RIA) was used. Following gel filtration chromatography on a Sephadex G-75 column, a major peak of immunoreactive alpha-MSH-sized material was detected. On the average there was 338 +/- 72 pmol (SD) of immunoreactive alpha-MSH per lungfish pars intermedia (n = 3). Following gel filtration the immunoreactive alpha-MSH was further analyzed by reverse-phase high-performance liquid chromatography (HPLC). Three peaks of immunoreactivity were detected. These peaks were designated Peaks 1, 2, and 3. The retention times of these peaks corresponded to, respectively, mammalian ACTH(1-13)amide, N-acetyl-ACTH(1-13)-amide, and N,O-diacetyl-ACTH(1-13)amide. Peaks 2 and 3 represented approximately 95% of the immunoreactive alpha-MSH recovered. Analysis of immunoreactive Peaks 2 and 3 by cation-ion-exchange indicated that both peaks had a net charge of +3 at pH 2.5. Since O-acetyl groups are sensitive to high pH, Peak 3 was incubated for 1 hr at 37 degrees in 0.01 N NaOH, pH 12. Under these conditions, Peak 3 eluted with the same retention time as untreated Peak 2. Collectively, these results indicate that Peaks 2 and 3 correspond to mono- and diacetylated lungfish alpha-MSH, respectively.

Isolation of alpha-melanotropin from the pars intermedia of the larval amphibian, Ambystoma tigrinum

The effect of background adaptation on the steady-state levels of alpha-melanotropin in the pars intermedia of the larval amphibian. Ambystoma tigrinum, was investigated. Acid extracts of pars intermedia obtained from light-adapted and dark-adapted animals were analyzed by radioimmunoassay following Sephadex gel filtration chromatography, reverse-phase HPLC, and Sulfopropyl Sephadex cation-ion-exchange chromatography. For both background adaptation conditions similar results were obtained. The major form of alpha-melanotropin present in the pars intermedia has the following properties: (1) an apparent molecular mass of 1.5 kDa; (2) a net charge at pH 3.5 of +4; and (3) a retention time following reverse-phase HPLC similar to that of synthetic ACTH(1-13)amide. In dark-adapted animals a minor form of alpha-melanotropin which has a net charge of +3 at pH 3.5 was also detected. The latter form represented approximately 10% of the total alpha-melanotropin immunoreactivity in the pars intermedia of dark-adapted animals. These results strongly suggest that the predominant form of alpha-melanotropin in the pars intermedia of larval A. tigrinum is a nonacetylated ACTH(1-13)amide-like polypeptide.

Steady-state analysis of alpha-melanotropin in the pars intermedia of Anolis carolinensis: effect of background adaptation

The steady-state levels of alpha-melanotropin-stimulating hormone (alpha-MSH)-related peptides were examined in the pars intermedia of the reptile Anolis carolinensis as a function of background adaptation. After a 7-day period, the content of immunoreactive alpha-MSH-related material in the pars intermedia of light-adapted animals was approximately fourfold higher than that of animals maintained on a dark background for the same period. The immunoreactive alpha-MSH-related material present in the pars intermedia of light-adapted and dark-adapted animals was separately analyzed by gel filtration chromatography, reverse-phase HPLC, and cation-exchange chromatography. For light-adapted animals the major form of alpha-MSH had an apparent molecular weight of 1.5 kDa and a net charge of +4 at pH 3.5. Following reverse-phase HPLC this material eluted as a single peak of immunoreactivity with a retention time distinct from that of both mammalian ACTH(1-13)amide and N-acetyl-ACTH(1-13)amide. For dark-adapted animals a peak of alpha-MSH-sized material with an apparent molecular weight of 1.5 kDa was also detected. Following reverse-phase HPLC analysis this material eluted as an apparent single peak of immunoreactivity with a retention time distinct from that of the mammalian standards. Subsequent analysis of this major HPLC peak by cation-exchange chromatography revealed the presence of at least two forms of immunoreactive alpha-MSH. These forms differed in relative proportions. The major peak of immunoreactivity had a net charge of +4, whereas the minor peak had a net charge of +3. The +3 immunoreactive form was not detected to any appreciable degree in light-adapted animals.

Expression of two proopiomelanocortin genes in the pituitary gland of Xenopus laevis: complete structures of the two preprohormones

A number of cDNA clones corresponding to Xenopus POMC mRNA was isolated from a cDNA library constructed from Xenopus pituitary polyadenylated RNA. Characterization of the cDNA inserts revealed two groups of structurally different proopiomelanocortin mRNAs, indicating that two proopiomelanocortin genes are expressed to virtually the same level in Xenopus pituitary glands. From the mRNA structures the complete amino acid sequences of the two Xenopus preproopiomelanocortins could be deduced. Comparison with proopiomelanocortin mRNA and protein sequences from other species shows regions of high homology (including the portion of the prohormone located N-terminally of gamma-melanophore-stimulating hormone) and regions of extremely low homology (including the signal sequence).

Proportions of mono- and diacetylated forms of alpha MSH in individual neurointermediate lobe extracts of Cyprinus carpio L

The proportions of the mono- and diacetylated forms of alpha MSH in individual carp neurointermediate lobe (NIL) extracts, as assessed by HPLC and RIA, fluctuate within a narrow range (mono/mono- + diacetyl alpha MSH = 3-14%). These results obtained on individuals of different sex and age show that the diacetylated form predominates in the NIL of all individuals where it represents in the mean 90% of the acetylated forms. The relatively low fluctuation of the proportions of the acetylated forms suggests that the possibility for a physiological modulation of the diacetylation step may be limited in this species of fish.

Separation and partial characterization by high-performance liquid chromatography and radioimmunoassay of different forms of melanocyte-stimulating hormone from fish (Cyprinidae) neurointermediate lobes

From neurointermediate lobe (NIL) extracts of two species of Cyprinidae, Carassius auratus and Cyprinus carpio, several peptides were separated by high-performance liquid chromatography (HPLC) on a C18 muBondapak column eluted with a methanol/acetic acid/triethylamine mixture. Monitoring all fractions by radioimmunoassay (RIA) with an antibody against melanocyte-stimulating hormone (MSH) C terminal gave positive reactions for fractions 7, 11-12, 15-16, 23-24, and 25-27. For further characterization, the elution positions of these peaks were compared to those of known synthetic reference substances. Peak 7 elutes in the same position as oxidized alpha MSH, whereas peak 15-16 matches the elution position of des-acetyl alpha MSH and 23-24 that of alpha MSH. The product from peak 26-27 has several characteristics of the diacetylated form of alpha MSH: its immunoreactivity in RIA, its sensitivity to weak bases and to HCl and its mass spectrum which is identical with that of mammalian diacetyl alpha MSH. In both species, the diacetylated form is predominant in the intracellular pool. This study establishes the coexistence of three different forms of alpha MSH, a des-acetylated, monoacetylated, and diacetylated in the cyprinid NIL extracts.

The mechanism of N-terminal acetylation of proteins

N alpha-acetylation is almost exclusively restricted to eukaryotic structural proteins. As a rule it is a post-initiational process, requiring the presence of the enzyme N alpha-acetyltransferase and the acetyl donor acetylcoenzyme A. N alpha-acetyltransferases appear to have a narrow substrate specificity, which is very similar for enzymes from different tissues and species. Amino acids predominantly present at the N terminus of N alpha-acetylated proteins are alanine, serine, and methionine. The occurrence of these residues is apparently a prerequisite for acetylation. The region following these amino acids is also important. If methionine is at the N terminus, the second position is always occupied by a strongly hydrophilic amino acid. Two- and three-dimensional structural characteristics of the protein do not seem to play a major role in N alpha-acetylation. Up to now the exact function for N alpha-acetylation is not known.

Further characterization of the major forms of reptile beta-endorphin

Biosynthetically labeled reptile intermediate pituitary beta-endorphin-sized material was fractionated by SP-Sephadex ion exchange chromatography into two major opiate-active forms which eluted at 0.28 M NaCl and 0.32 M NaCl, respectively; the 0.32 M form of reptile beta-endorphin (mw = 3500), serves as the precursor for the 0.28 M form of reptile beta-endorphin (mw = 3200), (Dores and Surprenant, 1983). Analysis of tryptic digests of these reptile beta-endorphins by paper electrophoresis at pH 3.5 and gel filtration on a Sephadex G-15 column indicated that there are two tyrosine residues, two arginine residues and one methionine residue in reptile beta-endorphin. Furthermore, the NH2-terminal tryptic peptide of both reptile beta-endorphins is approximately nine amino acids in size and contains tyrosine, methionine and arginine. Analyses of chymotryptic/protease digests of the [3H]tyrosine-labeled NH2-terminal tryptic peptide analyzed by descending paper chromatography revealed that the NH2-terminal tyrosine of reptile beta-endorphin is not alpha-N-acetylated. A second tyrosine-containing tryptic peptide was detected in the COOH-terminal region of reptile beta-endorphin; however this tryptic peptide differs in the two forms of reptile beta-endorphin in terms of size and net charge at pH 3.5. These differences account for the apparent molecular weight differences and distinct ion exchange properties of the 0.28 M and 0.32 M forms of reptile beta-endorphin. Thus in the reptile intermediate pituitary the principal post-translational mechanism for modifying beta-endorphin is COOH-terminal proteolytic cleavage.

Biosynthesis of multiple forms of beta-endorphin in the reptile intermediate pituitary

Fractionation of the beta-endorphin-sized material from freshly dissected reptile intermediate pituitaries by ion exchange chromatography on sulfopropyl Sephadex (SP) revealed at least three distinct forms of immunoreactive beta-endorphin. These forms eluted at 0.25 M NaCl, 0.28 M NaCl, and 0.32 M NaCl and represent respectively, 6%, 65% and 29% of the total immunoreactivity. Only the 0.28 M NaCl peak and the 0.32 M NaCl peak exhibited naloxone reversible opiate bioactivity when tested in the isolated guinea pig ileum bioassay system; taking into account the molar amount of immunoreactive peptides the 0.32 M NaCl peak was 6 fold more potent than the 0.28 M NaCl peak. Intermediate pituitaries in culture were incubated with either [3H]tyrosine, [3H]arginine, or [35S]methionine for periods up to 24 hours and beta-endorphin-sized peptides were prepared by immunoprecipitation and gel filtration. Fractionation of the labeled beta-endorphin-sized peptides by ion exchange chromatography yielded profiles nearly identical to the immunoassay analyses of freshly dissected tissue. Further analysis of the major labeled forms of reptile beta-endorphin by chromatography on Sephadex G-50 equilibrated in 6 M guanidine HCl indicated that the 0.32 M NaCl peak had an apparent molecular weight of 3500 +/- 100 and the 0.28 M NaCl peak had an apparent molecular weight of 3200 +/- 100. Furthermore, pulse/chase experiments showed that the 0.32 M NaCl peak was the precursor for the 0.28 M NaCl peak. These results coupled with the relative opiate bioactivities of the major argue that the principal post-translational modification of reptile beta-endorphin is COOH-terminal proteolytic cleavage.

Three types of alpha-melanocyte-stimulating hormone: bioactivities and half-lives

Three types of alpha-melanocyte-stimulating hormone (alpha MSH) that differ in the acetyl status of the N-terminal serine have been found in the neurointermediate lobe of the pituitary gland and in the brain: desacetyl alpha MSH, which lacks an acetyl group; monoacetyl alpha MSH, in which the amino group of the serine is acetylated; and diacetyl alpha MSH, in which both amino and hydroxy groups of the serine are acetylated. We compared the lipolytic and melanotropic actions of these three peptides, and their rates of disappearance from plasma, both in vitro and in vivo. The following differences were found. a) For in vitro lipolytic actions on rabbit adipose tissue slices, the potencies differed according to the order diacetyl = monoacetyl greater than desacetyl. On rabbit isolated adipocytes, however, the three peptides were equipotent. b) For in vivo lipolytic action in the rabbit, not only potency but also kinetics differed. Diacetyl alpha MSH had the slowest onset, longest duration, and greatest potency. The desacetyl variant had the quickest onset, shortest duration, and least potency. c) The half-life for elimination from rabbit plasma both in vitro and in vivo was shortest for the desacetyl form and longest for the diacetyl peptide. d) For in vitro melanotropic effect on frog skin, kinetics of action were the same for all three peptides, but potency differed according to the order diacetyl = monoacetyl greater than desacetyl. Thus acetylation of alpha MSH alters lipolytic and melanotropic potencies in vitro and lipolytic potency and kinetics in vivo. These differences result in part from the fact that acetylation slows the degradation of the tridecapeptide both inside and outside the circulation.

Evidence for an opiomelanotropin acetyltransferase in the rat pituitary neurointermediate lobe

Results of this study demonstrate two distinct forms of acetyltransferase activity which will acetylate alpha-MSH. These acetyltransferases are distinguished by pH optima, subcellular distribution and sensitivity to magnesium and several solubilizing detergents. A general acetyltransferase, as characterized in the rat pituitary anterior lobe and lens, has a pH optima of 7.4 and is inhibited by magnesium. Subcellular fractionation of anterior and neurointermediate lobes revealed that this acetyltransferase is primarily localized in the cytosol fraction of these tissues. An alpha-MSH acetyltransferase (MAT) and a beta-endorphin acetyltransferase (EAT) have a pH optima of 6.0-6.6, are inhibited by detergents, and are specifically localized in the secretory granules of the neurointermediate lobe. Comparative studies of MAT and EAT suggest a single enzyme responsible for the acetylation of opioid and melanotropin peptides, and we term this enzyme opiomelanotropin acetyltransferase (OMAT).

Distribution of an enzyme which acetylates alpha-melanocyte stimulating hormone in rat brain and pituitary gland and effects of arcuate nucleus lesions

This study describes the distribution of an alpha-melanocyte stimulating hormone (alpha-MSH) acetyltransferase (MAT) in rat brain and pituitary gland. Highest activities of MAT were found in the neurointermediate lobe of the pituitary gland with the anterior lobe containing slightly less. Within the brain, lowest MAT activities were measured in the hypothalamus, the region which contained the highest concentrations of alpha-MSH. Relatively high enzyme activities of MAT were measured in the hippocampus, cortex and cerebellum--regions with very low alpha-MSH concentrations. The fact that MAT activity levels did not parallel alpha-MSH concentrations indicates that MAT was not solely localized to alpha-MSH synthesizing neurons or endocrine cells. Furthermore, arcuate nucleus lesions which depleted brain alpha-MSH failed to deplete MAT activity. Although MAT was not solely localized to alpha-MSH synthesizing cells, it may have functional significance for alpha-MSH acetylation due to compartmentalization with alpha-MSH in alpha-MSH synthesizing endocrine cells and neurons. Alternatively a second regionally specific MAT may exist.

Preparation of tritiated alpha-melanotropin with high specific radioactivity

alpha-Melanotropin (alpha-MSH) was iodinated and 3,5-diiodotyr2-alpha-MSH was isolated by reverse phase high performance liquid chromatography (HPLC). Catalytic dehalogenation of the diiodo derivative in the presence of tritium resulted in the formation of 3,5-ditritiotyr2-alpha-MSH. The tritiated peptide was purified by ion exchange and partition chromatography. The radioactive peptide was found to be homogeneous and identical to alpha-MSH by paper electrophoresis and HPLC. The tritiated alpha-MSH stimulated lipolysis in rabbit adipocytes nearly as well as alpha-MSH. The specific radioactivity of tritiated alpha-MSH was 42 Ci/mmol or 73% of the theoretical value.