Enzymatic determination of 1-14C-glucose in pig plasma

Summary A simple and cheap one-step enzymatic method has been developed for the determination of 1-¹⁴C-glucose in plasma. C-1 of glucose is cleaved off as CO₂ by treatment with hexokinase, glucose-6-phosphate dehydrogenase, and 6-phosphogluconic dehydrogenase. True 1-¹⁴C-glucose activity is then calculated as the difference between total radioactivity and radioactivity remaining after enzyme treatment and evaporation. The reaction is shown to be quantitative and specific, thus eliminating both labelled metabolites and label recycled to other positions in glucose. Two different types of pig experiments show that 1-¹⁴C-glucose, when determined by this method, is as irreversible a tracer as the commonly used 3-³H-glucose.     

Enzymatic conversion of proteins to glycoproteins.

The enzymatic transfer of the oligosaccharide moiety from an oligosaccharide-lipid to denatured forms of three secretory proteins--ovalbumin, alpha-lactalbumin, and ribonuclease A--has been demonstrated utilizing a membrane fraction from hen oviduct. Based on a survey of 10 proteins denatured by sulfitolysis, the presence of the tripeptide sequence -Asn-X-Thr-Ser- (X represents a variable amino acid) appears to be necessary but not sufficient for the protein to serve as acceptor in vitro. The results of this investigation also suggest that unfolding of the polypeptide chain is required in order to expose sites for carbohydrate attachment.     

Enzymatic reactions involved in the repair of oxidized proteins

Proteins are the targets of reactive oxygen species, and cell aging is characterized by a build-up of oxidized proteins. Oxidized proteins tend to accumulate with age, due to either an increase in the rate of protein oxidation, a decrease in the rate of oxidized protein repair and degradation, or a combination of both mechanisms. Oxidized protein degradation is mainly carried out by the proteasomal system, which is the main intracellular proteolytic pathway involved in protein turnover and the elimination of damaged proteins. However, part of the oxidative damage to cysteine and methionine residues, two amino acids which are highly susceptible to oxidation, can be repaired by various enzymatic systems that catalyze the reduction of cysteine disulfide bridge, cysteine-sulfenic and -sulfinic acids as well as methionine sulfoxide. The aim of this review is to describe these enzymatic oxidized protein repair systems and their potential involvement in the decline of protein maintenance associated with aging, focusing in particular on the methionine sulfoxide reductases system.     

An improved fluorometric assay of rat serum and plasma converting enzyme

The most sensitive nonradiometric routine assay for angiotensin-converting enzyme (ACE) activity uses fluorometry to detect His-Leu released from Hip-His-Leu. Our results indicate that, in contrast to human serum, rat serum and plasma contain large and variable amounts of dipeptidase activity that lead to a subestimation of the ACE activity measured in 0.1 M potassium phosphate buffer, pH 8.3, containing 0.3 M NaCl, the most commonly used assay for human serum and tissue ACE. We describe and validate an assay for 1 to 10 microL rat and human serum or plasma using 5 mM Hip-His-Leu in 500 microL of 0.4 M sodium borate buffer, pH 8.3, containing 0.9 M NaC1 at 37 degrees C that reduced the subestimation error to less than or equal to 3% (rat serum) and less than or equal to 0.1% (human serum) and increased the ACE activity twofold to threefold. The Km and Vmax are reported for rat serum ACE (Hip-His-Leu) and dipeptidase (His-Leu) in borate buffer and phosphate buffer. Rat serum ACE hydrolysis of Hip-His-Leu measured by fluorometry correlated (r = 0.99, p less than 0.05) with the hydrolysis of angiotensin I measured by high-performance liquid chromatography. A direct method based on amino acid analysis is described for evaluating the dipeptidase error of complex mixtures such as tissue extracts and other physiological fluids. We have found that the assay can be used to measure ACE activity in 25 samples (in duplicate) in 2 hours with small intraassay (2.2%) and interassay (3.9%) coefficients of variation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Recombinant plasma proteins

For almost 50 years, the fractionation of human plasma has been the sole possible source of a wide range of therapeutic proteins--such as coagulation factors, anticoagulants, immunoglobulins, and albumin--essential to the treatment of serious congenital or acquired bleeding or immunological diseases. In the last 20 years, the application of recombinant technologies to mammalian cell cultures has made possible--although with some limitations in productivity, costs, and immunogenic risks--to produce and commercialize complex plasma glycoproteins for human therapeutic applications and has opened the way to the development of new molecular entities. Today, the advanced exploration of alternative cell lines and enhanced cell culture systems, as well as the development of alternative expression technologies, such as transgenic animals, is opening a new era in the production of the full range of recombinant plasma protein therapeutics. In this review, we examine the achievements and ongoing challenges of the recombinant DNA technology as a platform for the production of plasma proteins and the advantages and limitations of such products compared to fractionated plasma proteins.     

Steroid-binding proteins in primate plasma

We used immunological techniques to compare the serum corticosteroid-binding globulins (CBG) and testosterone-estradiol-binding globulins (TeBG) of Old World primates (man, chimpanzee, cynomologus, and rhesus), New World monkeys (squirrel and owl), and prosimians (galago and lemur). Four different antihuman TeBG antisera could not differentiate human and chimpanzee TeBG and recognized the galago and lemur TeBG as similar as well as the rhesus and cynomologus TeBG, as similar. Western blots of serum subjected to sodium dodecyl sulfate gel electrophoresis, with detection by an anti-TeBG antiserum, showed similar patterns of distribution of the two molecular species of TeBG for all of the New World primates and the owl monkey. The abundance of the two TeBG species was reversed in squirrel monkey serum, while lemur and galago displayed only a single band. Four different antihuman CBG antisera grouped together the CBGs of human and chimpanzee, rhesus and cynomologus, and lemur and galago. The squirrel monkey has a CBG with a markedly decreased affinity for cortisol; all four antisera perceived its CBG as much more immunologically distant from the human protein than that of the owl monkey. Indeed, three of the four antisera grouped squirrel monkey CBG with that of the prosimians, while one antiserum saw squirrel monkey CBG as even more distant from the human protein than the CBG of the primitive primates, the prosimians.     

Enzymatic modification of proteins with a geranylgeranyl isoprenoid.

The prenylation of several proteins involved in oncogenesis and signal transduction plays an essential role in regulating their biological activities. Two distinct isoprenoids are known to be involved in this modification, the 15-carbon farnesyl and 20-carbon geranylgeranyl groups. Thus far, identified farnesylated proteins contain methionine or serine at the COOH terminus, while those modified by geranylgeranyl end in leucine. This report describes the characterization of an enzyme activity that transfers the geranylgeranyl group to candidate proteins. The enzyme, termed a "protein geranylgeranyltransferase," exhibits a marked preference for substrate proteins that contain leucine at the COOH terminus. In fact, the enzyme will efficiently modify a normally farnesylated protein, Ha-ras, if its COOH-terminal amino acid is switched from serine to leucine. Additional studies characterize this enzyme and suggest that it is responsible for the geranylgeranyl modification of a number of GTP-binding proteins (or their subunits) that contain a consensus prenylation sequence ending in leucine.     

Enzymatic properties of purified Escherichia coli uvrABC proteins.

The cloned uvrA and uvrB genes of Escherichia coli K-12 were amplified by linkage to the PL promoter of plasmid pKC30. The uvrC gene was amplified in the high-copy-number plasmid pRLM 24. The three gene products (purified in each case to greater than 95% purity) and ATP are required to effectively incise UV-damaged DNAs. The uvrABC proteins bind tightly to damaged sites in DNA, requiring the initial attachment of the uvrA protein in the presence of ATP before productive binding of the uvrB and uvrC proteins. Using a cloned tandem double insert of the lac p-o region as a damaged DNA substrate for the uvrABC complex and analyzing the incision both 5' and 3' to each pyrimidine dimer, we found that one break occurs 7 nucleotides 5' to a pyrimidine dimer and a second break is made 3-4 nucleotides 3' from the same pair of pyrimidines in the dimer. No such breaks are found in the strand complementary to the dimer. The size of the incised fragment in the DNA suggests that incision may be coordinated with excision reactions in repair processes.     

Steroid hormone-binding proteins in blood plasma

Two types of proteins form dissociable complexes with the circulating steroid hormones in blood serum: albumin, the most abundant plasma protein, and the highly specific glycoproteins that occur in low concentrations: corticosteroid-binding globulin (CBG or transcortin), sex steroid-binding protein (SBP) and progesterone-binding globulin (PBG). Albumin interacts with low affinity, predominantly by hydrophobic bonding; the affinity constants of the steroid complexes with the specific globulins are higher by several orders of magnitude. CBG and SBP have molecular weights similar to that of albumin; PBG appears to be larger. A fairly general characteristic of these steroid-binding glycoproteins is their tendency to polymerize. Rat CBG forms dimers, tetramers and octamers upon removal of steroid from the complex; stoichiometric recombination with corticosterone completely reverses the polymerization. Estrogenic hormones increase the level of CBG and of SBP; androgenic hormones have the opposite effect. A CBG-depressing influence of corticosteroid hormones has been observed in the rat; adrenalectomized rats have increased CBG activity. The thyroid-stimulating hormone of the pituitary is required for the estrogen-induced rise of CBG synthesis in the rat; the effect is mediated by the thyroid gland. In accordance, thyroidectomy reduces the CBG activity whereas administration of thyroxin results in an increase. In contrast to all other species investigated, guinea pig serum contains separate proteins specific for binding of corticosteroid (CBG) and of progesterone (PBG). PBG levels increase about hundred-fold in pregnancy. PBG is a glycoprotein of χ₂- or β-globulin mobility with an approximate molecular weight of 100,000 and an association constant for progesterone-binding higher than any of the other steroid-serum protein complexes. Association with serum proteins suppresses the biological activity of steroid hormones. Bound hormone in blood constitutes an inactive pool which protects the hormone from metabolic and chemical alterations and provides, by reversible dissociation, a buffer against sudden changes in active hormone concentrations.     

Investigating diversity in human plasma proteins

Plasma proteins represent an important part of the human proteome. Although recent proteomics research efforts focus largely on determining the overall number of proteins circulating in plasma, it is equally important to delineate protein variations among individuals, because they can signal the onset of diseases and be used as biological markers in diagnostics. To date, there has been no systematic proteomics effort to characterize the breadth of structural modifications in individual proteins in the general population. In this work, we have undertaken a population proteomics study to define gene- and protein-level diversity that is encountered in the general population. Twenty-five plasma proteins from a cohort of 96 healthy individuals were investigated through affinity-based mass spectrometric assays. A total of 76 structural forms/variants were observed for the 25 proteins within the samples cohort. Posttranslational modifications were detected in 18 proteins, and point mutations were observed in 4 proteins. The frequency of occurrence of these variations was wide-ranged, with some modifications being observed in only one sample, and others detected in all 96 samples. Even though a relatively small cohort of individuals was investigated, the results from this study illustrate the extent of protein diversity in the human population and can be of immediate aid in clinical proteomics/biomarker studies by laying a basal-level statistical foundation from which protein diversity relating to disease can be evaluated.