Low Molecular Mass Species Research Articles

Phosphorylation of Nuclear Proteins in Myelinating Oligodendrocytes and Its Control by Cyclic AMP

Oligodendroglial nuclei isolated from rat brains at different stages of myelinogenesis (10, 18, and 30 days of age) were incubated with [gamma-32P]ATP and extracted with 0.75 M perchloric acid to yield a fraction of nonacidic chromatin proteins. The protein extracts were then analyzed by polyacrylamide gel electrophoresis. The phosphorylation pattern of these proteins was found to be different for different age groups. In 10-day-old rat oligodendrocytes the most extensive phosphorylation occurred in low molecular mass species (less than 30 kDa), in contrast to fractions obtained from 18- and 30-day-old rat oligodendrocytes which showed a significantly higher labeling of the proteins with molecular masses greater than 30 kDa. The phosphorylation of the latter species was greatly stimulated by the presence of cyclic AMP in the incubation media. The results suggest that the phosphorylation of specific nuclear proteins, which may play a regulatory role at different stages of oligodendroglial maturation and myelinogenesis, may be at least partially modulated by intracellular cyclic AMP.

Molecular Chaperon Produced by an Intracellular Symbiont1

Symbionin, that is selectively produced by an intracellular symbiont harbored by the aphid bacteriocyte, is structurally homologous to the Escherichia coli groEL protein, a heat shock protein functioning as a molecular chaperon. It was shown that symbionin has ATPase activity and, in the presence of Mg-ATP, is converted into lower molecular mass species. Like the groEL protein, symbionin was able to reconstitute dimeric ribulose 1,5-bisphosphate carboxylase/oxygenase holoenzyme from its unfolded subunits in vitro, suggesting that this protein functions as a molecular chaperon in the endosymbiont. The groES-homologous protein did exist in the endosymbiont, but its amount was small relative to that of symbionin.

Oxidative damage to synovial fluid from the inflamed rheumatoid joint detected by 1H NMR spectroscopy.

The glycosaminoglycan hyaluronate (hyaluronic acid, hyaluronan) [l] is a linear repeating disaccharide, @-D-glucuronyl-p-D-Nacetylglucosamine, of very high molecular mass (>4 X 106) which is a major component of the proteoglycan aggregates required for the functional integrity of extra-cellular matrices such as articular cartilage [2]. Hyaluronan is continually secreted in its unaggregated form by the type II synoviocytes and is largely responsible for the very high viscosity of kneejoint synovial fluid [3]. It has previously been demonstrated that in patients with rheumatoid arthritis or other inflammatory joint diseases, synovial fluid hyaluronate is depolymerized with a corresponding modification in the viscoelastic properties of synovial fluid and an increase in the synovial level of low-molecular-mass hyaluronate-derived saccharides [4]. This fragmentation is presumed to occur by the action of reactive oxygen radical species (RORS) [5] since (1) hyaluronidase activity is absent from both normal and inflammatory synovial fluid samples [6], and (2) there is currently a large quantity of experimental evidence implicating the involvement of these chemically-reactive oxygen metabolites in the pathogenesis of inflammatory joint diseases [7-91. In addition to the presence of potential oxygen radical generating systems within the joint, this hypothesis is also supported by the demonstration of hyaluronate depolymerization by such systems in vivo as evidenced by decreases in apparent hyaluronate molecular mass and viscometric parameters. We have previously suggested that hypoxicreperfusion injury during perambulatory motion of the inflamed rheumatoid joint is a mechanism which perpetuates the deleterious production of oxygen-derived radical species within the joint, giving rise to the chronicity of the inflammation [9]. Much of the toxicity of RORS to biological systems is attributable to the highly reactive hydroxyl radical (*OH) [lo], the formation of which appears to be mediated by low-molecular-mass iron chelates. In this communication we present evidence for RORS-mediated damage to knee-joint synovial fluid in patients with inflammatory synovitis. Nuclear magnetic resonance (NMR) spectroscopy has been extensively used for investigating the metabolic profile of body fluids [ll], particularly for monitoring the status and levels of low-molecular-mass endogenous components. We have employed high field proton Hahn spin-echo NMR spectroscopy to detect and characterize a series of lowmolecular-mass species derived from the attack of radiolytically-generated RORS on hyaluronate. We have also utilized this technique to assess RORS-mediated oxidative damage to synovial fluid hyaluronate in vivo during hypoxic-reperfusion injury. In addition, the

Fingerprinting of near-homogeneous DNA ligase I and II from human cells. Similarity of their AMP-binding domains.

DNA ligases play obligatory roles during replication, repair, and recombination. Multiple forms of DNA ligase have been reported in mammalian cells including DNA ligase I, the high molecular mass species which functions during replication, and DNA ligase II, the low molecular mass species which is associated with repair. In addition, alterations in DNA ligase activities have been reported in acute lymphocytic leukemia cells, Bloom's syndrome cells, and cells undergoing differentiation and development. To better distinguish the biochemical and molecular properties of the various DNA ligases from human cells, we have developed a method of purifying multiple species of DNA ligase from HeLa cells by chromatography through DEAE-Bio-Gel, CM-Bio-Gel, hydroxylapatite, Sephacryl S-300, Mono P, and DNA-cellulose. DNA-cellulose chromatography of the partially purified enzymes resolved multiple species of DNA ligase after labeling the enzyme with [alpha-32P]ATP to form the ligase-[32P]AMP adduct. The early eluting enzyme activity (0.25 M NaCl) contained a major 67-kDa-labeled protein, while the late eluting activity (0.48 M NaCl) contained two major labeled proteins of 90 and 78 kDa. Neutralization experiments with antiligase I antibodies indicated that the early and late eluting activity peaks were DNA ligase II and I, respectively. The three major ligase-[32P]AMP polypeptides (90, 78, and 67 kDa) were subsequently purified to near homogeneity by elution from preparative sodium dodecyl sulfate-polyacrylamide gels. All three polypeptides retained DNA ligase activities after gel elution and renaturation. To further reveal the relationship between these enzymes, partial digestion by V8-protease was performed. All three purified polypeptides gave rise to a common 22-kDa-labeled fragment for their AMP-binding domains, indicating that the catalytic sites of ligase I and II are quite similar, if not identical. Similar findings were obtained from the two-dimensional gel electrophoresis of their AMP-binding domains in the trypsin-digested protein fragments. The results also suggested that these isozymes have been derived from the same primordial DNA sequence or from the same precursor protein. The purification scheme and the data obtained will be instrumental for the further elucidation of the biological roles of various DNA ligases from human cells.

DNA topoisomerase I from human placenta

In this study we report that human placenta is an excellent source of DNA topoisomerase I. The enzyme can be isolated in the fully intact 100 kDa form although lower molecular mass species are also observed. Occasionally, the enzyme can be resolved into two peaks of activity by chromatography on phosphocellulose. As expected, the enzyme promotes marked cleavage of DNA in response to the anticancer drug camptothecin. Because of this property and the ready availability of human placenta, the enzyme should prove to be useful in the development and testing of new anticancer drugs that target topoisomerase I.

The earliest site of iodination in thyroglobulin is residue number 5.

In most highly structured native proteins, as well as in thyroglobulin, the reactivity in vitro of the various tyrosyl residues toward iodine is widely different. The present work demonstrates that of nearly 70 tyrosyl residues present in rat thyroglobulin, there is one, residue number 5 from the NH2-terminal end, which has in vivo the highest affinity toward iodine, being the first one to be iodinated. In fact, when 6-(n-propyl)-2-thiouracil (PTU)-treated, iodine-deficient animals were injected with 125I and killed shortly after, we isolated from thyroid glands poorly iodinated thyroglobulin (about 1 iodine atom/thyroglobulin molecule), nearly 90% of the radioactivity of which was found as monoiodotyrosine. Although CNBr cleavage of this protein gave several fragments after gel electrophoresis only one of these, with apparent mass 27,000 Da, contained 125I. This fragment was isolated and fully characterized. Twelve cycles of automated Edman degradation were performed; the sequence found, i.e. N-I-F-E-X-Q-V-X-A-Q-X-L, indicated that the 27,000-Da fragment is the NH2 terminus of thyroglobulin. This portion of the polypeptide chain contains several tyrosyl residues which may well all be potentially involved in the early iodination of the protein. The observation that the removal of seven amino acids from the NH2 terminus is accompanied (at the fifth step) by the total disappearance of radioactivity in the resulting shortened peptide suggested that the fifth residue was the only one iodinated under these conditions. A second, more quantitative experiment was performed on thyroglobulin obtained from 6-(n-propyl)-2-thiouracil-treated animals whose death was postponed 24 h after the injection of 125I. In this case the radioactivity was found not only in a single CNBr fragment (27,000 Da) but also in other discrete species of lower molecular mass. The mixture of these peptides was subjected to seven steps of manual Edman degradation. Fragments before and after partial degradation were run in parallel on a polyacrylamide gel and the distribution of 125I compared. Besides some change in the background, the two profiles were identical except for the absence of the 27,000-Da species. This proves that all the 125I present in the 27,000-Da species was localized at the fifth residue, the same site at which the hormone molecule is preferentially synthesized under normal conditions. This result is not unexpected and is in accord with the known properties of thyroglobulin which has a polypeptide chain designed for efficient synthesis of the hormone even at low levels of iodination.

Identification and quantification of apolipoprotein D in normal human urine

Apolipoprotein D has been identified in normal human urine, using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, followed by immunoblotting with monospecific antibodies. Urinary apolipoprotein D appeared as a main 33,000 u protein together with a minor fraction corresponding to its partially deglycosylated species of lower molecular mass. No high molecular mass forms of apolipoprotein D naturally occurring in plasma could be detected. The apolipoprotein D mean +/- SD concentration assayed with rocket immunoelectrophoresis, in urine samples from nine apparently healthy normal men, was 1.4 +/- 1.0 mg/L (range: 0.2-3.0 mg/L). Among the plasma apolipoproteins, apolipoprotein D behaves uniquely as regards its excretion in urine; the other apolipoproteins belonging to the A, B, D and E groups, although of low molecular masses, are present, at most, in trace amounts in normal urine.

The osteoclast functional antigen, implicated in the regulation of bone resorption, is biochemically related to the vitronectin receptor.

We have defined the structure of the Osteoclast Functional Antigen (OFA) by immunological and biochemical means. OFA is an abundant surface antigen in human and animal osteoclasts and has been characterized previously by monoclonal antibodies 13C2 and 23C6, one of which mimicks the inhibitory activity of calcitonin on osteoclastic bone resorption. By the following criteria we show that OFA is a member of the integrin family of extracellular matrix receptors and is identical, or at least highly related, to the vitronectin receptor (VNR) previously isolated from placenta and melanoma cells. Immunoprecipitation analysis demonstrates that OFA from osteoclasts and a monkey kidney cell line Vero is a heterodimeric molecule of 140 kD (alpha chain) and 85 kD (beta chain) under nonreducing conditions; on reduction at least one low molecular mass (alpha') species (of approximately 30-kD size) is released, resulting in a 120/100-kD dimer. Immunoblots of OFA isolated from osteoclasts and Vero cells and VNR purified from placenta and probed with heterosera to OFA and monoclonal antibodies to platelet gp111a (VNR beta chain) show immunological cross-reactivity between the alpha chains of OFA and VNR and the use of gp111a as a beta chain by both. OFA from Vero cells binds to an Arg-Gly-Asp containing peptide (GRGDSPPK) isolating a heterodimer recognized by anti-OFA monoclonal antibodies, 13C2 and 23C6. Immunohistochemical analysis showed a similar tissue distribution in humans for the antigen recognized by anti-OFA antibodies, a monoclonal antibody, LM142, raised to melanoma VNR, polyclonal antibodies to the placental VNR and a monoclonal antibody to the presumptive VNR beta chain, platelet glycoprotein 111a. Finally, NH2 terminal amino acid sequencing showed that the amino-terminus of the monkey alpha chain was identical in the 12 assigned residues to that of human VNR alpha chain. The beta chain sequence of OFA differed at least 1 (and up to 4) positions from platelet gp111a (VNR beta) in the first 18 amino acids sequenced. These, and other, data provide the first indication of a function for the VNR and suggest that cell-cell and cell-extracellular matrix interactions involving integrins may play an important role in bone physiology.

Cation binding effects on the pH, thermal and urea denaturation transitions in alpha-lactalbumin.

The binding of monovalent (Na+, K+) and divalent (Ca2+, Mg2+) cations to bovine alpha-lactalbumin at 20 and 37 degrees C has been studied by means of intrinsic protein fluorescence. The values of apparent binding constants for these ions obtained at 37 degrees C are about one order of magnitude lower than those measured at 20 degrees C. Urea and alkali (pH greater than 10) induce unfolding transitions which involve stable partially unfolded intermediates for all metal ion-bound forms of alpha-lactalbumin. Heating induces similar partially unfolded states. Nevertheless, the partially unfolded states induced by heating, urea, alkaline or acidic treatments are somewhat different in their tryptophan residue environment properties. The results have been interpreted in terms of a simple scheme of equilibria between metal-free and metal-bound forms in their native, partially unfolded and unfolded states. The scheme provides an approach to the quantitative interpretation of any transition equilibrium shift induced by a low molecular mass species able to be bound by a protein.