Rapid and sensitive high-performance liquid chromatographic method for the analysis of tryptophan, tyrosine and phenylalanine in biological samples

Circular dichroism studies of subtilisin Carlsberg immobilised on micron sized silica particles

Immobilised enzymes are widely used in industry, but the reasons for loss of activity of such biocatalysts are usually not known. We have used circular dichroism (CD) to investigate the structure of one such system, i.e., subtilisin Carlsberg (SC) immobilised on silica gel particles (60 microm). A number of technical problems have to be overcome in order to obtain appropriate data from which conclusions can be drawn. A rotating cell holder has been developed to avoid sedimentation of the silica particles during the collection of spectra. By moving the cell holder as close as possible to the detector window, the effects of differential scattering can be minimised. However, the effects of absorption flattening limit the extent to which reliable quantitative information on secondary structure content can be obtained from far UV CD studies. We have used an empirical approach based on absorbance units derived from the high-tension voltage to correct for absorption flattening effects. After applying the correction there was satisfactory agreement with the solution spectra. Comparison of the fresh and used (inactive) SC-silica gel spectra in organic media reveals substantial change in the secondary structure. Additional evidence for loss of native conformation is provided by the significant decrease in the near UV CD spectrum. These results for the first time clearly demonstrate the origin of enzyme instability in the immobilised state.

Proteomic methods applied to the analysis of immobilized biocatalysts

Methods adapted from proteomics can directly characterize proteins present in immobilized biocatalysts. Complete hydrolysis followed by HPLC analysis of Tyr and Phe estimates total protein bound, and is preferable to conventional difference methods, as tested with subtilisin Carlsberg on silica. This new method shows that various treatments give quantitative desorption of proteins immobilized by adsorption. Intact desorbed proteins may be analyzed by electrospray mass spectrometry. The Candida antarctica lipase B from Novozyme 435 was shown to be heavily glycosylated, while the lipase from Lipozyme RM IM was a mixture of four N-terminally truncated forms. Peptides from selective cleavage were analyzed by tandem mass spectrometry, leading to automatic identification of proteins present. A second major protein present in Lipozyme RM IM was thus found to be alpha-amylase from Aspergillus oryzae. These methods should be valuable complements to activity measurements in understanding immobilized enzyme activity and stability.

Human intestinal permeability of piroxicam, propranolol, phenylalanine, and PEG 400 determined by jejunal perfusion

PURPOSE

To determine the human jejunal permeabilities of compounds utilizing different transport mechanisms using a regional perfusion approach and to establish a standard procedure for determining drug permeability class to be used for the establishment of drug product bioequivalence standards.

METHODS

Six healthy male volunteers participated in this study. A multi-lumen perfusion tube was inserted orally and positioned in the proximal region of the jejunum. A solution containing piroxicam, phenylalanine, propranolol, PEG 400 and PEG 4000 was perfused through the intestinal segment at a rate of 3.0 ml/min. Perfusate samples were quantitatively collected every 10 minutes for two 100 minute periods with an intermediate wash out period to determine intra and intersubject variation.

RESULTS

The mean P(eff) (+/-SD) of piroxicam, phenylalanine, propranolol, and PEG 400 were 10.40 +/- 5.93, 6.67 +/- 3.42, 3.59 +/- 1.60, 0.80 0.46 x 10(-4) cm/sec, respectively. The coefficient of variation for the intersubject variability, first and second perfusion periods were: piroxicam, 60.5% and 57.1%; phenylalanine, 52.8% and 57.8%; propranolol, 62.1% and 44.6%; and PEG 400, 81.7% and 42.3%, indicating a slightly lower CV for the second perfusion period in the same subject. The intrasubject CV's between the two perfusion periods were: 19.4%, 21.3%, 23.6% and 41.0% respectively, indicating a smaller intraindividual variation for all compounds studied.

CONCLUSIONS

Piroxicam, a nonpolar drug exhibited the highest permeability of the compounds studied. The intrasubject CV was lower than the intersubject CV, indicating consistent permeability estimation within subjects. The methodology is useful for permeability estimation regardless of absorption mechanism and can be used to establish a consistent data base of human permeabilities for estimation of human drug absorption and for establishing the biopharmaceutic permeability class of drugs.