Search PubMed⌕ Search

Biomedical subjects

J L Bada

Publications and source records attributed to J L Bada.

At least 73 records · Page 4Linked to original sources
PubMed1987

Racemization and the origin of optically active organic compounds in living organisms.

The organic compounds synthesized in prebiotic experiments are racemic mixtures. A number of proposals have been offered to explain how asymmetric organic compounds formed on the Earth before life arose, with the influence of chiral weak nuclear interactions being the most frequent proposal. This and other proposed asymmetric syntheses give only slight enantiomeric excess and any slight excess will be degraded by racemization. This applies particularly to amino acids where half-lives of 10(5)-10(6) years are to be expected at temperatures characteristic of the Earth's surface. Since the generation of chiral molecules could not have been a significant process under geological conditions, the origins of this asymmetry must have occurred at the time of the origin of life or shortly thereafter. It is possible that the compounds in the first living organisms were prochiral rather than chiral; this is unlikely for amino acids, but it is possible for the monomers of RNA-like molecules.

Amino Acids↗
PubMed1985

The racemization rate of selenomethionine and methionine in yeast at 100 degrees C and neutral pH.

The rate of racemization of selenomethionine in yeast at 100 degrees C and pH 7.4 was determined using gas chromatography to separate the trifluoroacetyl-L-prolyl-D,L-selenomethionine methyl esters. The racemization half-life (i.e., the time required to attain a D/L ratio of 0.33) of selenomethionine was found to be similar to that of methionine and have a value of 19-20 days.

Chromatography, Gas↗
PubMed1984

The chemical conditions on the parent body of the Murchison meteorite: some conclusions based on amino, hydroxy and dicarboxylic acids.

Amino and hydroxy acids have been identified in the Murchison meteorite. Their presence is consistent with a synthetic pathway involving aldehydes, hydrogen cyanide and ammonia in an aqueous environment (Strecker-cyanohydrin synthesis). From the various equilibrium and rate constants involved in this synthesis, four independent estimates of the ammonium ion concentrations on the parent body at the time of compound synthesis are obtained; all values are about 2 x 10(-3) M. Succinic acid and beta-alanine have also been detected in the Murchison meteorite. Their presence is consistent with a synthesis from acrylonitrile, hydrogen cyanide and ammonia. Using the equilibrium and rate constants for this synthetic pathway, and the succinic acid/beta-alanine ratio measured in the Murchison meteorite, an estimate of the hydrogen cyanide concentration of 10(-3) to 10(-2) M is obtained. Since hydrogen cyanide hydrolyzes relatively rapidly in an aqueous environment (t1/2 < 10(4) yrs) this high concentration implies a period of synthesis of organic compounds as short as 10(4) years on the Murchison meteorite parent body.

Acrylonitrile↗
PubMed1984

Racemization of aspartic acid and phenylalanine in the sweetener aspartame at 100 degrees C.

The racemization half-lives (i.e., the time required to reach a D/L = 0.33) at pH 6.8 for aspartic acid and phenylalanine in the sweetener aspartame (L-aspartyl-L-phenylalanine methyl ester) were determined to be 13 and 23 hours, respectively, at 100 degrees C. Racemization at this pH does not occur in aspartame but rather in its diketopiperazine decomposition product. Our results indicate that the use of aspartame to sweeten neutral pH foods and beverages that are then heated at elevated temperature could generate D-aspartic acid and D-phenylalanine. The nutritive consequences of these D-amino acids in the human diet are not well established, and thus aspartame should probably not be used as a sweetener when the exposure of neutral pH foods and beverages to elevated temperatures is required. At pH 4, a typical pH of most foods and beverages that might be sweetened with aspartame, the half-lives are 47 hours for aspartic acid and 1200 hours for phenylalanine at 100 degrees C. Racemization at pH 4 takes place in aspartame itself. Although the racemization rates at pH 4 are slow and no appreciable racemization of aspartic acid and phenylalanine should occur during the normal use of aspartame, some food and beverage components could conceivably act as catalysts. Additional studies are required to evaluate whether the use of aspartame as a sugar substitute might not in turn result in an increased human consumption of D-aspartic acid and D-phenylalanine.

Aspartame↗
PubMed1984

Investigations of in vivo methionine racemization in mammalian tissues.

The racemization rate of methionine at pH 7.4 and 100 degrees C was determined to be similar to that of aspartic acid. However, analyses of dentin and ocular lens nucleus samples from known age Rhesus monkeys showed that unlike aspartic acid, methionine does not undergo in vivo racemization in mammalian proteins.

Aging↗
PubMed1978

Aspartic acid racemization in heavy molecular weight crystallins and water insoluble protein from normal human lenses and cataracts.

High D/L aspartic acid ratios are observed in heavy molecular weight aggregates and in water-insoluble protein extracted from whole lenses and nuclear and cortical regions. Purified alpha-, beta-, and gamma-crystallins have low D/L ratios. Fractionation of urea-solubilized material from the water-insoluble protein yields four molecular weight classes of proteins. Fractions representing crosslinked material or apparently degraded products have high D/L ratios. Racemization within lens proteins may contribute to formation of the water-insoluble fraction seen in aging lenses and cataracts.

Adolescent↗
PubMed1977

Seroepidemiology of the poliovirus in Monrovia (Liberia).

Neutralizing antibodies against poliovirus have been studied in 265 persons in Monrovia (Liberia). 34% of the individuals studied possessed antibodies against the three polioviruses, 23% against two, 26% against one; the remaining 17% showed none. The three poliovirus antibodies were evenly distributed amongst the population, although there was an indication that antibodies to polio 2 occurred more commonly in females than in males. The distribution of the antibodies did not seem to be influenced by place of residence, source of drinking water or social class. Significant variations in the immunological state according to age were noted. The study of concordance and discordance of antibody titre in the blood of neonates and their mothers revealed that the lower percentage of titre concordance for polio 3 has no statistical value. The low titre in the neonates (an average for each poliovirus scarcely above 1:16) is a sign of insufficient titre in the adults. The need for a massive antipoliomyelitis campaign among infants and adult women is stressed.

Age Factors↗
PubMed1977

Considerations on the role of aspartic acid racemization in the aging process.

Racemization of aspartyl residues in human dentine and enamel proteins has been shown to occur at a rate which corresponds to an enrichment in the D-aspartic acid content of 0.1% per year. This rate can be used to calculate the ages of living people or the in vivo lifetimes of slowly turned over proteins. We present stereochemical arguments for conformational changes in proteins as a consequence of racemized amino acid residues. In metabolically stable proteins, this phenomenon may play some part in the aging process. In renewed proteins, where certain factors may accelerate racemization, conformational changes induced by racemization could regulate protein degradation.

Adolescent↗