Amino acid sequence of a variant pro-form of insulin-like growth factor II

Human serum contains, in addition to the "classical" 7.5-kDa insulin-like growth factors (IGFs) I and II, small amounts of larger IGF-II. A 10-kDa IGF-II was isolated by gel filtration, immunoaffinity chromatography, and reversed-phase HPLC. Upon amino acid sequence determination, a substitution of Cys-Gly-Asp for Ser-33 was found as well as a COOH-terminal extension of 21 residues (E peptide). These sequence differences suggest that 10-kDa IGF-II is a precursor of a variant IGF-II. Since the substitution is not located at a known intron/exon hinge region, the finding of this variant IGF-II is evidence for the presence of more than one gene for IGF-II.

Partial purification and characterization of somatomedin from sheep serum

A rapid, high yield preparative technique for the isolation of sheep somatomedin is reported. Purification of biologically active somatomedin from the 60% ammonium sulfate precipitate of sheep serum was accomplished using three gentle fractionation steps. Biological activity during purification was monitored using the rat adipocyte nonsuppressible insulin-like activity (NSILA) assay. A stepwise pH elution (pH 2.85, 3.5, 4.5, and 6.0) from SP-Sephadex resulted in the elimination of more than 99% of the serum proteins and a 500-fold enhancement of biological activity. The active fraction eluted at pH 6.0 and was further fractionated on Sephadex G-50 (fine) chromatography at pH 2.85. This resulted in about a 10,000-fold enhancement of activity over serum activity. The most active fractions from Sephadex chromatography were further separated on reverse phase HPLC in 0.1% trifluoroacetic acid using a linear gradient of 24-60% acetonitrile. The biological activity of the final preparation was enhanced 61,000- to 182,000-fold over that of serum (mean, 93,000-fold) when assayed in the NSILA assay. Protein yield was estimated to be 467 micrograms/liter serum. In addition to the NSILA activity, the final preparation demonstrated dose-dependent sulfation factor activity in the embryonic chick pelvic leaflet bioassay. Sheep somatomedin was active at physiological levels in both bioassays. Analysis of the somatomedin preparation by sodium dodecyl sulfate-electrophoresis at pH 8.8 showed that it was homogeneous by this criterion. The activity eluted from Sephadex G-50 was estimated to have a molecular size of 6900. Two Coomassie blue-stained bands were present in the final sheep somatomedin preparation after polyacrylamide gel electrophoresis at pH 3.2. Our purification process is a rapid, high yield technique which yields a polypeptide fraction enriched in NSILA and somatomedin-like activity. The molecular size and biological activity in the NSILA and sulfation factor assays suggest that our sheep NSILA is analogous to somatomedins purified from other species of animals.

Insulin-like growth factor II (IGF II) in human brain: regional distribution of IGF II and of higher molecular mass forms

Twenty-four distinct areas of human brain were analyzed for the presence of insulin-like growth factor (IGF). As reported for cerebrospinal fluid, only IGF II-like immunoreactivity, but no significant amounts of IGF I-like immunoreactivity, could be found. Upon gel permeation chromatography, two to five distinct size classes were separated on the basis of their immunoreactivity. The smallest component had an apparent molecular mass of 7.5 kDa, identical to the one of purified IGF II from human serum. Radioimmunoassays and a bioassay also gave results indistinguishable from those of serum IGF II. The highest amounts of IGF II-like immunoreactivity occur in the anterior pituitary--namely, 20-25 pmol equivalents/g of wet weight. This is up to 100 times more than in most other brain regions analyzed. The higher molecular mass immunoreactive species were partially characterized. After immunoaffinity purification, the 38- and 26-kDa species are active in a bioassay. Specific IGF-binding protein activity could be shown after purification of the 38- and 26-kDa species on an IGF-affinity column. The 13-kDa species released significant amounts of 7.5-kDa material. The results are interpreted as evidence for the presence of IGF II synthesized locally in human brain. The structure of the larger forms of IGF II-like immunoreactive material as well as the function of IGF II in brain are not yet known.

Efficient secretion and purification of human insulin-like growth factor I with a gene fusion vector in Staphylococci

A novel approach for production of small polypeptides, using a staphylococcal protein A vector, is described. This system is used to express, secrete and purify human insulin-like growth factor I (IGF-I). A fusion protein consisting of protein A and IGF-I is recovered in high yield by passing the culture medium through an IgG affinity column. Using site-specific mutagenesis an acid labile asp-pro cleavage site was introduced at the fusion point between the two proteins. The protein A "tail" can thereby be removed from the affinity purified fusion protein by chemical cleavage releasing biologically active IGF-I molecules.

Efficient purification of somatomedin-C/insulin-like growth factor I using immunoaffinity chromatography

Somatomedin-C/insulin-like growth factor I was purified from human plasma using a monoclonal antibody affinity column. Combining immunoaffinity chromatography with standard protein purification methods resulted in an overall recovery of 18%. The 35 micrograms of somatomedin-C/insulin-like growth factor I purified from 500 ml of plasma appeared as a single band when analyzed by polyacrylamide gel electrophoresis and could be used in radioimmunoassay and receptor binding studies.

Partial purification of a specific inhibitor of the insulin-like growth factors by reversed-phase high-performance liquid chromatography

We report here preliminary data using reversed-phase high-performance liquid chromatography for the purification of a specific inhibitor (a molecular weight 16,000-18,000 protein) of the insulin-like growth factor (IGF) or somatomedin family. Crude inhibitor prepared from Cohn fraction IV-1 of human serum was first partially purified using an IGF/CH-Sepharose 4B affinity column. Following elution of the bound inhibitor and resuspension in 0.1% aqueous trifluoroacetic acid (mobile phase A), it was injected (100 microliter; 2.0 mg protein) onto a Brownlee Aquapore RP-300 column. Application of a linear gradient from 0% to 100% mobile phase B (45% isopropanol-0.1% trifluoroacetic acid) resulted in elution of two peaks of inhibitor activity between 31% and 34% isopropanol associated with a major homogeneous protein peak and a minor heterogeneous protein peak. No inhibitor was recovered when an acetonitrile gradient was used instead of isopropanol, indicating that the inhibitor is very hydrophobic. These data suggest that high-performance liquid chromatography offers a simple procedure for the potential purification of IGF inhibitor(s) from normal human serum.

Chemistry, structure, and function of insulin-like growth factors and their receptors: a review

Human serum contains two classes of somatomedins (Sm) with similar three-dimensional structures and mass (i.e., approximately 7500 kdaltons (kd), but that can be distinguished from each other by their isoelectric point (pI) values. Those with alkaline pI's include the presumably identical molecules of insulin-like growth factor I (IGF-I), SmA, SmC, and basic Sm. In humans, the neutral-acidic class of Sm is represented by IGF-II. The sera of rats and other animals contain similar classes of Sm. In normal children and adults, plasma levels of both IGF-I and -II are under growth hormone control, although only elevated concentrations of the former can be positively correlated with adolescent skeletal growth. Elevated blood levels of IGF-II or an embryonic form of it, however, are correlated with fetal development of rats, sheep, and possibly in humans. As evidenced from competitive radioreceptor binding studies and affinity-labelling techniques, receptors that bind insulin and IGF-I are immunologically, structurally, and functionally related: monoclonal and polyclonal antibodies to insulin receptors recognize determinants on receptors that bind IGF-I; both receptors have 140-, 95-, and possibly 45-kd subunits that are interchain disulfide-bonded to form glycoprotein complexes of relative masses (Mr) 300 000 - 400 000; and occupancy of the 140-kd subunits by hormone or growth factor stimulates the phosphorylation of tyrosine residues on the 95-kd subunit by receptor-associated protein kinases.(ABSTRACT TRUNCATED AT 250 WORDS)

Proteolytic conversion of insulin-like growth factors to an acidic form(s)

The relative amounts of the various forms of bioassayable insulin-like growth factors (IGF) isolated from human serum or serum fraction Cohn IV-1 depend on the purification procedure. With acid gel filtration or acid/ethanol extraction as the initial step, IGF-II (pI approximately 6.5) was the most abundant (40-70%) followed by somatomedin A (pI approximately 7.4; 15-23%), an acidic form of insulin-like activity (ILA pI 4.8) (13-21%) and IGF-I (pI approximately 8.5; 5-27%). If, however, pH 5.5 ion-exchange chromatography on SP-Sephadex was used prior to acid gel filtration, the acidic pI 4.8 form was the major (greater than 90%) species recovered and was accompanied by a quantitative loss of the other IGF species. This suggested a possible conversion of IGF-I, somatomedin A and/or IGF-II to the acidic ILA pI 4.8 form(s) during the SP-Sephadex procedure. Further experiments indicated that differences in the yields of ILA pI 4.8 were not due simply to differences in the initial pH conditions of the various methods (i.e. acid versus neutral), although exposure to pH 9.7 (a pH experienced during elution of IGF activity from the SP-Sephadex) did appear to play a role. The involvement of the carrier protein in the conversion process was tested by subjecting carrier-free IGF-I and IGF-II to the SP-Sephadex procedure. No conversion of the free forms to ILA pI 4.8 occurred. To examine the possible role of proteinase in the conversion of IGFs to ILA pI 4.8, SP-Sephadex chromatography was performed in the presence of a broad spectrum proteinase inhibitor. The IGF distribution pattern obtained closely resembled the 'normal' pattern seen with acid gel filtration, indicating that proteinase inactivation had prevented conversion to ILA pI 4.8. These data suggest that proteolytic conversion of IGF-I, somatomedin A and IGF-II to more acidic ILA pI 4.8 form(s) (i) occurs during SP-Sephadex chromatography, (ii) is not prevented simply by prior acid exposure, and (iii) takes place only when IGF-I and -II are in their high-Mr carrier-bound forms. Since IGF-I and IGF-II, although homologous, have unique amino acid sequences, the conversion of both IGFs implies that at least two acidic ILA forms exist. Nevertheless, because ILA pI 4.8 retains the full spectrum of IGF bioactivities in vitro, and significant quantities are present in normal human serum (21%), it would suggest that proteolytic conversion of IGF-I, somatomedin A and IGF-II to ILA pI 4.8 in vivo may be a physiologically significant event.

Insulin-like growth factor characteristics of an acidic non-suppressible insulin-like activity

The biological activities of an acidic form of non-suppressible insulin-like activity (ILA pI 4.8) have been studied. ILA pI 4.8 was isolated from Cohn fraction IV-1 of human serum by pH 5.5 ion-exchange chromatography on SP-Sephadex. Carrier-bound ILA was eluted at pH 9.7 and then sequentially gel chromatographed in 1% formic acid on Sephadex G-75 and Bio-Gel P-30. The low-Mr (7000) active material was subjected to flat bed isoelectric focusing. Overall recovery was 87 munit of insulin equivalents/100 g of Cohn fraction IV-1, with a specific activity in the range 4-10 munit/mg of protein, representing a purity of 1-6%. This material has been tested in a variety of insulin-like growth factor (IGF)/somatomedin assay systems. It stimulated, in a dose-related manner, [14C]glucose conversion into lipid by isolated rat adipocytes, 35SO4(2-) incorporation into weanling rat costal cartilage and [3H]thymidine incorporation into DNA of cultured human fibroblasts. Like IGF-I and -II, ILA pI 4.8 was able to inhibit degradation of 125I-insulin by crude homogenates of rat liver. In addition, the biological activity of ILA pI 4.8 was completely suppressible by a recently described inhibitor of IGF-I and IGF-II. ILA pI 4.8 was able to compete, in a parallel manner, with 125I-IGF-I and 125I-IGF-II and, at higher doses, with 125I-insulin in a placental radioreceptor assay. No cross-reactivity was seen in a radioimmunoassay for IGF-I and -II C-peptides, but at higher concentrations parallel displacement was observed in a somatomedin C/IGF-I radioimmunoassay using two different antisera. These data indicate that ILA pI 4.8 does possess many of the biological activities previously reported for the IGFs. Since ILA pI 4.8 does occur naturally in serum, it would appear reasonable to tentatively include it as one of the IGF/somatomedin family.

Isolation of insulin-like growth Factors I and II from human plasma

Insulin-like growth factors I and II have been isolated from Cohn fraction IV-1 of human plasma using gel permeation chromatography, ion exchange chromatography, reversed phase chromatography, isoelectric focusing (IEF) and high performance liquid chromatography(HPLC). IGF I of specific activity 89 U/g, as measured by the isolated rat adipocyte assay, and IGF II, of specific activity 78 U/g, were obtained in yields of 16 micrograms and 34 micrograms respectively per 100g of Cohn fraction. Although this process yields IGF I which is contaminated with IGF II (due to the relatively large amount of the latter present in the original plasma), the IGF II preparations appear to be relatively free from IGF I. This separation was mainly achieved with IEF since the two factors elute close together on HPLC. Nevertheless, HPLC is important for their subsequent purification. The process is thus especially suitable for the preparation of IGF II and appears to give better yields than those obtained by earlier methods which used acid-ethanol extraction, gel permeation chromatography and polyacrylamide gel electrophoresis.

Extraction of cartilage sulfation inhibitors and somatomedins from rat liver

Both cartilage sulfation inhibitors and somatomedins were identified in extracts from rat liver. After extraction at pH 7.4 (neutral extract), cartilage inhibitors were found in neutral liver extracts from normal, diabetic, fasted, and hypophysectomized rats [61 +/- 4%, 68 +/- 3% 60 +/- 6%, and 62 +/- 7% below assay buffer (mean +/- SEM), respectively; all P less than 0.001 below buffer]. Exposure to 80 C or pH 2.4, inactivated the inhibitors in all samples. After gel filtration on Sephadex G-100, pH 7.4, cartilage inhibitors in the neutral extracts had molecular sizes similar to those of inhibitors in the diabetic rat serum (mol wt, greater than 45,000). After the inhibitors were removed with neutral buffer extraction, reextraction at pH 2.4 (neutral/acid extracts) revealed somatomedins only in the livers of normal rats. Cartilage stimulation by somatomedins in neutral/acid extracts was linear and parallel to stimulation by normal serum. After gel filtration on Sephadex G-75, pH 2.4, somatomedins in neutral/acid extracts from normal rat liver were found to have an apparent molecular weight larger than that of serum somatomedins (mol wt, approximately 25,000-45,000), consistent with recent findings for hepatic somatomedins. The data indicate that both somatomedins and inhibitors can be extracted from rat liver.

Tissue concentrations of somatomedin C: further evidence for multiple sites of synthesis and paracrine or autocrine mechanisms of action

We have validated a method for extracting and measuring the tissue content of somatomedin C (Sm-C)/insulin-like growth factor I (IGF-I), a growth-hormone-dependent, growth-promoting peptide. The Sm-C content of tissue extracts was strongly growth-hormone dependent because most of the tissues studied from hypophysectomized rats contained significantly less Sm-C than normal tissues. The intraperitoneal administration of ovine growth hormone (oGH) to hypophysectomized rats caused tissue extractable Sm-C to increase in kidney, liver, lung, heart, and testes. Tissue Sm-C responses to oGH were maximal after 12 hr, 6 hr before the maximal increment in serum. In liver and lung, the tissue Sm-C response to various doses of oGH fit linear regression models, and the doses of oGH needed to increase the Sm-C are in the range of those required to increase protein synthesis. Although these results do not exclude the possibility that the somatomedins act by hormone-like endocrine mechanisms, they add support to the concept that these peptides act through autocrine or paracrine mechanisms, being produced at multiple sites and acting at or near their sites of production.

Isolation and partial sequence analysis of rat basic somatomedin

Rat basic somatomedin (Sm) was prepared with an improved purification scheme from the pooled sera of Wistar Furth rats previously inoculated with cells from the pituitary tumor MStT/W15. Amicon hollow fiber diafiltration facilitated the processing of large batches of serum, eliminating the necessity of running smaller portions of material on gel filtration columns to achieve the same qualitative separation. The yield from this technique was excellent. Basic rat Sm was separated from a C3ades Arg component of complement by a narrow pH range isoelectric focusing step. Subsequent chromatography on carboxymethyl-cellulose and Sephadex resins was very efficient, providing a 100-fold purification, with a recovery of Sm activity of approximately 65%. The final product was judged to be pure by a variety of criteria. Structural analysis of this material has demonstrated that the amino-terminal sequences of rat basic Sm and human insulin-like growth factor I are strikingly similar, confirming the homology proposed earlier on the basis of receptor cross-reactivity.

Extraction of somatomedin activity from rat liver

Biologically active somatomedins were evaluated after extraction from rat tissues. Somatomedin activity was measured by bioassay as stimulation of sulfate uptake by hypophysectomized rat costal cartilage. Somatomedin activity was found in the liver after extraction with acid but not with neutral media. Activity was not found in acid extracts of spleen, muscle, brain, or kidney. While acetic acid liver extracts from normal rats had significant activity, extracts from hypophysectomized rats had no activity; treatment with GH in vivo restored hepatic activity to normal. After gel filtration on Sephadex G-75 in 1 M acetic acid, somatomedin activity recovered from liver extracts was of a somewhat higher molecular weight (approximately 30,000) than activity in serum fractions (approximately 8,000). Extract somatomedins produced dose-response stimulation of cartilage parallel to that by serum somatomedins; the total hepatic content of somatomedin activity was over twice as great as the total serum activity. Cartilage-inhibiting (somatomedin inhibitory) activity was also found in extract fractions; the apparent molecular weight was approximately 50,000. Hypophysectomized rats given liver extract somatomedins in vivo exhibited modest but significant increases in costal cartilage sulfate uptake, an index of growth activity. These observations of a substantial hepatic content of somatomedin activity which is 1) GH dependent, 2) not found in other tissues, and 3) biologically active both in vitro and in vivo support the concept that serum somatomedins originate in the liver.

Somatomedin-like peptide(s) isolated from fetal bovine cartilage (cartilage-derived factor): isolation and some properties

Fetal bovine cartilage contains a polypeptide(s) that has somatomedin-like effects on rat and rabbit costal chondrocytes in culture. This factor, named the cartilage-derived factor, was extracted from fetal bovine cartilage, fractionated with acetone, and purified by gel filtration on Toyopearl HW 55-F in 4 M guanidine hydrochloride, preparative isoelectric focusing, and subsequent gel filtration on Toyopearl HW 55-F in 1 M formic acid. The resulting preparation, which focused in the neutral pH region and eluted from a Toyopearl column in a fraction with apparent Mr 10,000--11,000, appeared homogenous by NaDodSO4 gel electrophoresis. The purified preparation markedly enhanced not only proteoglycan synthesis but also DNA synthesis in rabbit costal chondrocytes and, on a protein basis, it was 1000 times more active than insulin and 1,000,000 times more active than fetal calf serum in stimulating proteoglycan synthesis.

Basic peptide with insulin-like activity in human serum

A peptide with insulin-like activity was isolated from human plasma. In the purification, insulin-like activity (ILA) was monitored by radioreceptor assay for insulin, using human placental membrane and [125I]-insulin. ILA was extracted from Cohn fraction III with acid-ethanol and chromatographed on Sephadex G-75 in 1% formic acid. When the active fractions were subjected to ion-exchange chromatography with CM-cellulose, the ILA was adsorbed to the column at pH 5.0 and was eluted with a gradient of ammonium acetate. The chromatographic behavior of the ILA was not identical to that of somatomedin A as determined by radioreceptor assay (RRA) for the latter. On isoelectric focusing of the ILA from the CM-cellulose column, insulin-like activity was distributed over a wide pH range. The ILA that was focused at pH 7.5-9.0 was further purified by gel filtration on Sephadex G-50 in 1 M acetic acid. The specific activity of the basic ILA was approximately 200 mU insulin equivalent /mg protein. The apparent molecular weight of the material was estimated to be 7,000. It stimulated [14C]-glucose oxidation in rat epididymal fat cells and had sulfation activity in chick chondrocytes. Furthermore, the basic ILA had a potent mitogenic activity in Balb/c-3T3 cells. Thus, the basic ILA is qualified as an 'insulin-like growth factor' (IGF). It is obviously different from somatomedin A, and may be closely related to IGF-I or somatomedin C. Subsequently, RRA for the basic ILA was developed in serum concentration of the basic ILA were determined. The serum concentrations of the basic ILA were high in acromegalics and low in patients with hypopituitarism. Thus, the basic ILA is entitled to be one of the IGFs.

Purification of somatomedin-C from human plasma: chemical and biological properties, partial sequence analysis, and relationship to other somatomedins

Somatomedin-C (SM-C) was purified from Cohn fraction IV of human plasma by a series of steps which included cation-exchange chromatography, gel filtration, isoelectric focusing, and reverse-phase high-pressure liquid chromatography. The peptide isolated contained 10,142 units/mg, as judged by a radioimmunoassay, representing a 780,000-fold purification from native plasma. The isolated peptide was basic (pI 8.1-8.5) and was judged to be no less than 90% pure. The best fit for integral amino acids was obtained with 78 residues. Limited sequence data were obtained on the N terminus and on five fragments obtained by tryptic digestion after blocking the lysine residues. Three of these fragments and the N terminus could be aligned with portions of insulin-like growth factor I (IGF-I). Of the 25 residues so aligned, 22 were identical with IGF-I. In addition, two tryptic fragments were obtained which are not present in the sequence of IGF-I. SM-C and IGF-I produced identical curves of displacement in radioimmunoassay and radioreceptor assays for SM-C and in an insulin radioreceptor assay. The potencies of SM-C and IGF-I in these assays differed significantly from IGF-II, somatomedin-A, and several preparations of multiplication stimulating activity. At a concentration of 1 ng/mL, SM-C stimulated the progression of Balb/c 3T3 cells into DNA synthesis and when injected in vivo restored mitosis in lens epithelium of hypophysectomized frogs.

The purification and partial characterization of human somatomedin C

Somatomedin C was purified from a Cohn IV fraction of human plasma by acid release followed by dialysis, ultrafiltration, Sephadex G-50 chromatography, and isoelectric focusing between pH 7 to 11. The resulting preparation, which focused at pH 8.6, was essentially free of insulin contamination and appeared homogeneous by acid gel chromatography and sodium dodecyl sulfate gel electrophoresis. The somatomedin C was purified about one-half-million-fold with a yield of 7%. The purified preparation had an apparent molecular weight of 7,400 with no indication of intermolecular disulfide bonds. Its amino acid composition revealed a lysine-rich peptide containing significant amounts of histidine and methionine. No free sulfhydryl was found. It stimulated in vitro cartilage sulfation and DNA synthesis of hypophysectomized rats. In the presence of hypopituitary serum, it increased cartilage sulfation of embryonic chick. Somatomedin C competed potentially with [125I]iodoinsulin for insulin receptors on human placenta cell membranes. [125I]Iodosomatomedin C apparently binds preferentialy to a high affinity placental receptor which is different from the insulin receptor.

Improved recovery of somatomedin activity from plasma by prolonged formic acid extraction

Recovery of somatomedin activity from plasma fractions was increased approximately 5-fold by prolonged extraction of isolates with formic acid. Extraction for 72 hours enhanced the yields of active peptides in the 6,000, 8,000 and > 20,000 dalton range. Yields of smaller peptides were similar following short (2 hrs) and long (72 hrs) extraction periods. Active fractions obtained after prolonged extraction possessed sulfation activity and insulin-receptor affinity comparable to somatomedin isolated during short extraction with formic acid.