Aggrecan turnover in human articular cartilage: use of aspartic acid racemization as a marker of molecular age

Aggrecan is a key component of the cartilage matrix. During aging, many changes occur in its composition and structure; in particular, there is an increase in the proportion of lower molecular weight monomers and of the "free" binding region. An important question has been whether these changes represent alterations in biosynthesis or whether they are due to the accumulation with age of the partially degraded fragments of the originally synthesized large monomer. In the present work we have used an independent tool, viz., the extent of racemization of aspartic acid to study the molecular "age" of different buoyant density fractions of the aggrecan of human articular cartilage, as well as of isolated free binding region and link protein. By measuring the D/LAsp ratio of the different aggrecan species, we were able to establish directly the relative residence times of these molecules in the cartilage matrix and, in combination with compositional and structural analyses, to define their "history" and calculate some of the kinetics constants characterizing their turnover. The value of the turnover constant for the large monomer in fraction A1D1 is 0.206 per year, which corresponds to a half-life of 3.4 years, while the turnover constant for the free binding region is 0.027 per year, which corresponds to a half-life of 25 years. It is thus clear that the rate of formation and turnover of the large monomer is much more rapid than the final degradation of the free binding region fragments, which explains the accumulation of the latter in cartilage during aging.

Racemization of aspartic acid in the extracellular matrix proteins of primary and secondary dentin

The accumulation with age of D-aspartic acid in primary and secondary human dentin was determined. In primary dentin, the plot of the D/L Asp ratio vs. age was found to fit first-order kinetics in accordance with the literature. But the secondary dentin behaved in an irregular manner and showed, in the great majority of cases, significantly increased D/L Asp ratios. Possible reasons for these findings, such as differences in protein composition and/or in the prevailing temperature, are discussed.

Racemization of aspartic acid in human articular cartilage

The rate of racemization of aspartic acid was measured in young and aged human femoral head cartilage. Normal femoral heads were obtained at postmortem, osteoarthritic specimens at operations for total hip replacement. In order to distinguish between the aspartic acid racemization in collagen from that in proteoglycan (PG), in addition to native tissue, we tested cartilage specimens from which PG had been enzymatically removed. Preliminary results indicate that there is only a very slow collagen turnover in normal adult cartilage. The same is true of residual cartilage from osteoarthritic femoral heads, indicating no rapid repair except where osteophytes are formed. Native, PG-containing cartilage, whether normal or osteoarthritic was found to have unexpectedly high racemization rates.

Detection of aspartic acid enantiomers by chiral capillary gas chromatography. Determination of in vivo racemization and reduction of metal-induced background

Racemization of aspartyl residues in proteins is a post-translational process, related to ageing. A method is presented for the detection of aspartic acid enantiomers in protein hydrolysates, based on chiral capillary gas chromatography. It is fast, easy and preferable to the usual diastereomeric dipeptide technique. We present evidence that traces of metals that are extracted from the glassware during acidic hydrolysis are the main cause for high background racemization, which often troubles accurate measurements. Effective ways to reduce this background and its standard deviation to acceptable levels are discussed, and a mathematical approach to correct for background racemization is given. Hydrolysates of aged human eye lens proteins were used to demonstrate the enantiomeric separation.

Direct enantiomeric resolution of mephenytoin and its N-demethylated metabolite in plasma and blood using chiral capillary gas chromatography

A gas chromatographic method was developed for the determination of the R- and S- enantiomers of the anticonvulsant, mephenytoin, and its N-demethylated metabolite, 5-phenyl-5- ethylhydantoin ( PEH ), in plasma and blood. Direct enantiomeric separation of mephenytoin and its internal standard was obtained using a chiral capillary column ( Chirasil -Val) followed by nitrogen specific detection. However, resolution of the enantiomers of PEH and its internal standard required propylation at the 3-position of the hydantoin ring prior to analysis. Similar linear and reproducible standard curves were obtained from both plasma and blood over the concentration range 50 ng/ml to 5 micrograms/ml, and above 100 ng/ml the reproducibility was less than 8% (coefficient of variation). Pronounced stereoselective differences in the plasma concentration--time curves for both mephenytoin and PEH were observed in a normal subject who received a single oral dose of 300 mg racemic mephenytoin. The peak plasma level of S-mephenytoin was only one-fifth that of the R-enantiomer and its elimination half-life was less than 3 h compared to over 70 h for R-mephenytoin. Similarly, S- PEH levels were barely detectable whereas concentrations of R-metabolite steadily increased over 4-6 days before slowly declining.

Gas chromatographic separation of diastereoisomeric amino acid derivatives on glass capillaries. THE USE OF PENTAFLUOROPROPIONYL-AMINO ACID (+)-3-METHYL-2-BUTYL ESTERS

The separation of diastereoisomeric pentafluoropropionyl-D, L-amino acid (+)-3-methyl-2-butyl esters, including those derived from histidine, arginine and tryptophan, is achieved on glass capillaries. Separation factors of between 1.03 and 1.09 are observed for the derivatives of protein amino acids. The gas chromatographic separation of a mixture of common amino acids usually takes less than 1 h.

Interaction between asymmetric solutes and solvents. Diamides derived from L-valine as stationary phases in gas-liquid partition chromatography

The resolution of enantiomers of (+/-)-a-amino acid derivatives on optically active diamides derived from L-valine, R1CONHCH[CH(CH3)2]CONHR2, as gas chromatographic stationary phases was studied. The effects of the structure of R1 and R2 in the stationary phases, the structure of the racemic a-amino acid derivatives, temperature, etc., on the separation factors are reported. In the light of these and related results, a more probable mechanism of resolution is presented.

Gas chromatographic resolution of enantiomeric amphetamines and related amines. II. Effects of cyclic structures on diastereomer and enantiomer resolution

Proline and other cyclic amino acids were used as acylated acid chlorides for the resolution of enantiomers of amphentamine and related amines as diastereomers by gas chromatography. Variation of ring size or incorporation of a heteroatom into the ring of the cyclic resolving agent changed retention times and separation factors, depending on the racemic amine derivatized. Separations were achieved on a short, packed Carbowax 20M column. Because N-trifluoroacetyl L-prolyl chloride has proved especially effective for derivatization of amine enantiomers to give diastereomers which often may be resolved by gas chromatography, the proline ring was incorporated into a new optically active stationary phase. For the enantiomeric derivatives examined, the new proline ester phase gave separations essentially equivalent to those obtained on the commercially available phase, carbonyl bis-(L-valine isopropyl ester). The fact that resolution was achieved on the new phase is evidence that, as has been previously suggested, the ester carbonyls of this type of phase are the significant sites of interaction with the antipodes undergoing separation.

Present status of enantiomeric analysis by gas chromatography

The development of selective chiral solute-solvent systems showing separation of enantiomers by gas chromatography is reviewed. The enantiomeric analysis of alpha-amino-acids and its application to various problems, including notably the analysis of meteorites, is discussed. Some recent results with the highly efficient diamide phases, which permit resolution on packed columns, are presented. Some new ideas on the mechanism of resolution are briefly mentioned.

Gas chromatography of amino acids

This review summarizes all papers that have appeared on the gas chromatography of amino acids (including the iodoamino acids) and their enantiomers in the period 1956-mid-1974. It has been found that the methods used for analysis of amino acids can be divided into three classes: (1) degradative procedures and techniques for converting the amino acid into another chemical compound; (2) procedures based on esterification of the carboxyl group and derivatization of the a-amino and other reactive groups in at least two steps; and (3) procedures based on a simultaneous derivatization of the carboxyl and a-amino groups in one reaction medium. For the treatment of the amino acid or its alkyl ester, three approaches can be distinguished for the two latter cases, i.e., acylation, alkylation (including silylation) and condensation. Of the procedures used for the resolution of optical antipodes, two methods are discussed, namely analysis of diastereoisomers on optically inactive stationary phases and separation of enantiomers on optically active stationary phases.

Gas chromatographic resolution of enantiomeric amphetamines and related amines. I. Structural effects on some diastereomer separations

Eight structurally related enantiomeric primary amines including several amphetamines were resolved as distereomers by gas chromatography. Optically active acylated amino acid chlorides were reacted with the amine racemates to yield volative diastereomers which were chromatographed on polar and on nonpolar columns. Selectivity of diastereomeric pair separations was found to be dependent on substitution at the chiral centers of both the amino acids and the amines. Baseline resolution was achieved on a 6-ft. diethylene glycol succinate column at 200 degrees for seven of the eight amines. The polar stationary phases yielded higher relative separation factors than the nonpolar phase in all cases for the same chromatographic conditions. The effects of varying the perfluoroacyl group of the amino acid resolving agent from trifluoroacetyl to pentafluoropropionyl to heptafluorobutyryl was to significantly decrease the retention times without significantly decreasing these separation factors.