Switching substrate preference of thermophilic xylose isomerase from D-xylose to D-glucose by redesigning the substrate binding pocket

The substrate specificity of thermophilic xylose isomerase from Clostridium thermosulfurogenes was examined by using predictions from the known crystal structure of the Arthrobacter enzyme and site-directed mutagenesis of the thermophile xylA gene. The orientation of glucose as a substrate in the active site of the thermophilic enzyme was modeled to position the C-6 end of hexose toward His-101 in the substrate-binding pocket. The locations of Met-87, Thr-89, Val-134, and Glu-180, which contact the C-6-OH group of the substrate in the sorbitol-bound xylose isomerase from Arthrobacter [Collyer, C.A., Henrick, K. & Blow, D. M. (1990) J. Mol. Biol. 212, 211-235], are equivalent to those of Trp-139, Thr-141, Val-186, and Glu-232 in the thermophilic enzyme. Replacement of Trp-139 with Phe reduced the Km and enhanced the kcat of the mutant thermophilic enzyme toward glucose, whereas this substitution reversed the effect toward xylose. Replacement of Val-186 with Thr also enhanced the catalytic efficiency of the enzyme toward glucose. Double mutants with replacements Trp-139----Phe/Val-186----Thr and Trp-139----Phe/Val-186----Ser had a higher catalytic efficiency (kcat/Km) for glucose than the wild-type enzyme of 5- and 2-fold, respectively. They also exhibited 1.5- and 3-fold higher catalytic efficiency for D-glucose than for D-xylose, respectively. These results provide evidence that alteration in substrate specificity of factitious thermophilic xylose isomerases can be achieved by designing reduced steric constraints and enhanced hydrogen-bonding capacity for glucose in the substrate-binding pocket of the active site.

Synthesis, characterization, and biodistribution of [113mIn]TE-BAT: a new myocardial imaging agent

In order to develop a new myocardial perfusion agent, new lipid-soluble complexes containing a net charge of +1 were evaluated. Synthesis, radiolabeling, characterization, and biodistribution of a unique indium complex, [113mIn]TE-BAT (tetraethyl-bis-aminoethanethiol), are described. The complex formation between In +3 and TE-BAT ligand is rapid, simple, and of high yield (greater than or equal to 95%). This process is amenable to kit formulation. The complex has a net charge of +1 and an In/ligand ratio of 1:1. Biodistribution in mice shows higher heart uptake and longer retention as compared to 201TI. This complex, when labeled with 111In, shows promise as a possible tracer for myocardial perfusion imaging.

Human proapolipoprotein A-I: development of an antibody to the propeptide as a probe of apolipoprotein A-I biosynthesis and processing

In human plasma, apolipoprotein A-I is present as pro and mature isoproteins. The development of a highly specific antibody to the pro isoprotein of apoA-I has been difficult due to the close structural similarity between the pro and mature isoforms of apoA-I. To sermount this difficulty, a peptide was synthesized by the solid phase method which corresponded to the amino acid sequence present in the pro region of apoA-I. The synthetic peptide was coupled to serum albumin and the conjugate utilized to immunize rabbits for antibody production. Immunoblot analysis of purified proapoA-I and mature apoA-I revealed that the antibody was specific for the propeptide of apoA-I. Analysis of apoA-I in the plasma from a Tangier disease patient and newly secreted apoA-I from HepG2 cells clearly demonstrated the isoforms which contained the proisoprotein. The proapoA-I specific antibody should prove to be a useful tool in developing a radioimmunoassay for quantitation of the proisoprotein in plasma, isolation of proapoA-I from normal and dyslipoproteinemic subjects by immunoaffinity chromatography and in studies related to the synthesis and processing of apoA-I.

Identification of sequence homology between human plasma apolipoprotein B-100 and apolipoprotein B-48

The structural relationship between apolipoprotein B-100 (apo-B-100) and apolipoprotein B-48 (apo-B-48) has not been elucidated. A peptide fragment (MDB-18) of approximately 6 kDa was isolated from a tryptic digest of apo-B-100. The sequence of the first 22 amino acids of MDB-18 was determined by Edman degradation. A 15-residue peptide corresponding to this sequence was synthesized by the solid-phase method and was utilized to develop a sequence-specific polyclonal antibody. On immunoblot analysis, the antibody recognized both intact apo-B-100 and apo-B-48. In addition, preincubating the antibody with the synthetic peptide abolished the recognition of both apo-B-100 and apo-B-48. These data are interpreted as indicating that there is an amino acid sequence homology between apo-B-100 and apo-B-48. Since the MDB-18 peptide is located in the carboxyl region of the B-100 molecule, we propose apo-B-100 and apo-B-48 share a common carboxyl region sequence.

Isolation and characterization of apolipoproteins A-I, A-II, and A-IV

A number of different analytical techniques are now available for the isolation of apoA-I, apoA-II, and apoA-IV. The choice of a particular technique is dependent on the instrumentation available, and the quantity of isolated apolipoprotein required. The isolation and characterization of the separate isoforms and the precursor isoproteins of the individual apolipoproteins are detailed, and methods for the evaluation of the purity of the separate apolipoproteins presented. A method for the evaluation of apolipoproteins in plasma is now available which permits the identification of structural variants of plasma apolipoproteins in patients with dyslipoproteinemias.

Human proapoA-ITangier: isolation of proapoA-ITangier and amino acid sequence of the propeptide

The metabolic defect in Tangier disease is an increased catabolism of apoA-ITangier. The plasma concentration of proapoA-ITangier (apoA-I1 isoform) is increased in patients with Tangier disease. ProapoA-ITangier has been purified to homogeneity, and the amino acid sequence of the propeptide determined by automated Edman degradation. The propeptide sequence was Arg-His-Phe-Trp-Gln-Gln which is identical to the propeptide sequence of normal proapoA-I. These studies indicate that the increase in plasma proapoA-ITangier is not due to a structural defect in the propeptide sequence of proapoA-ITangier and a defect in conversion of proapoA-ITangier to mature apoA-ITangier. The increased catabolism of apoA-ITangier is due to a primary structural defect in mature apoA-ITangier.

Human plasma proapoA-I: isolation and amino-terminal sequence

Human apoA-I is synthesized as preproapoA-I, a 267 amino acid precursor apolipoprotein. PreproapoA-I initially undergoes intracellular co-translational proteolytic cleavage into proapoA-I. ProapoA-I is secreted from the cell and was isolated from thoracic duct lymph in the apoA-I1 isoform position. The amino-terminal sequence of proapoA-I isolated from human lymph revealed the presence of 6 additional amino acids, Arg-His-Phe-Trp-Gln-Gln, on the amino-terminal end of apoA-I consistent with the proapoA-I sequence determined by nucleic acid sequence analysis of cloned apoA-I. Our results indicate that proapoA-I is present in human plasma, and undergoes post-translational proteolytic cleavage to mature plasma apoA-I.

Isolation and partial characterization of a guinea pig serum apolipoprotein comigrating with apo-E on sodium dodecyl sulfate-polyacrylamide electropherograms

Study of guinea pig plasma lipoproteins has shown that they contain a polypeptide that comigrates with the arginine-rich polypeptide (apo-E) by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This comigrating polypeptide differs from apo-E in its amino acid composition, immunological cross-reactivities, electrophoretic mobility in urea polyacrylamide gel, and elution volume from Sephadex gel columns. It is present in very low density lipoproteins and low density lipoproteins from both control and cholesterol-fed guinea pigs.

Plasma cholesterol esterifying activity in guinea pigs

Plasma cholesterol esterifying activity has been measured in guinea pigs fed either a control diet or the same diet supplemented with 1% cholesterol. The extent of esterification was found to be similar in the cholesterol-fed and control guinea pigs and somewhat lower than in rats. The initial rate of esterification was also of the same magnitude as that found in rats and humans, and unaffected by dietary cholesterol if autologous plasma was used as substrate. However, LCAT activity from cholesterol-fed guinea pigs was significantly higher than that of control plasma when acting on either control or cholesterol-fed substrate. This suggests that dietary cholesterol increases the amount (or activity) of LCAT but that the substrate is unsuitable or that a necessary cofactor is present in limiting amounts. Heat treatment of guinea pig plasma seems to alter substrate availability to varying degrees. The implications of these findings in relation to substrate specificity and cofactor requirements of guinea pig LCAT are discussed.

Changes in the plasma lipoprotein-apoproteins of guinea pigs in response to dietary cholesterol

The major apoproteins from four plasma lipoproteins were isolated from control and cholesterol-fed guinea pigs. Apoproteins were studied by column chromatography, polyacrylamide gel electrophoresis, and amino acid analysis. Dietary cholesterol altered the plasma apolipoproteins mainly by an enrichment in the content of arginine-rich polypeptide (ARP) in all density fractions. This protein had a similar molecular weight (34 000), electrophoretic mobility, amino acid composition, and microheterogeneity as ARP reported in other mammalian species. The estimation of plasma concentration of ARP indicates a higher correlation coefficient with plasma unesterified cholesterol (r = 0.98) compared with cholesterol esters (r = 0.62).