Application of immobilized aspartate aminotransferase and immobilized aspartate aminotransferase-malate dehydrogenase coupled system to micro-assay of L-aspartic acid

Flow-injection analysis of amino acids and their metabolites by immobilized vitamin B6-dependent enzymes. Sensitive determination of L-aspartate, L-glutamate, 2-oxoglutarate, and oxaloacetate

Sensitive flow-injection analyses of aspartate, glutamate, 2-oxoglutarate, and oxaloacetate were developed. The analytes were enzymatically coupled with NADH which was monitored by light emission from immobilized bacterial bioluminescence enzymes. Aspartate (or oxaloacetate) was assayed on the basis of NADH consumption by introducing the sample through a coimmobilized aspartate aminotransferase-malate dehydrogenase column. The assay responded linearly from 100 pmoles to 5 nmoles per assay. Glutamate (2-oxoglutarate) was determined by formation of NADH in the glutamate dehydrogenase reaction. The measuring range for glutamate was from 10 pmoles to 100 nmoles per assay. The precision of the flow-injection method was generally excellent, and the sensitivities of the described assays were 100-1000-fold higher than with spectrophotometric methods. The immobilized enzyme preparations were stable for several months in storage, and the enzyme columns could be used for 600-800 analyses. Flow-injection analyses of amino acids and related compounds by NADH/bioluminescence-coupled reactions provide a sensitive, fast, and inexpensive assay method for a wide variety of purposes.

Immobilization of aspartate aminotransferase on agarose

Various methods for immobilization of aspartate aminotransferase (AspAT; from cytosolic fraction of pig heart) on agarose were tested. Aldehyde-, thiol-, and CNBr-activated agaroses were studied in detail. The capacity of the aldehyde support to firmly bind protein was less than 0.2 mg/ml, whereas the apparent remaining specific activity of the bound AspAT was high (50-63% of soluble AspAT). The maximum capacity of SH-agarose to bind enzymatic protein was 3 mg/ml; the apparent remaining activity was 30-40%, and the specific activity determined by Vmax was 51%. Chemical coupling on to thiol-agarose did not denature the enzyme, as 93% of protein and 83% of the activity were recovered after release of the enzyme from the support. Enzyme protein was quantitatively bound to CNBr-activated agarose (up to 10 mg/ml of the gel). The apparent specific activities were 27-35%, while the value calculated from Vmax was 46%. Active site-protecting agents within the CNBr-coupling were tested. Bromphenol blue increased the apparent specific activity to 60% and Vmax to 80% at 3-fold molar concentration at the active sites. Kinetic constants for immobilized preparations were determined.

Comparison of the coupling recoveries of immobilized aspartate aminotransferase. Specific activity, enzyme-bound coenzyme, and transaminationable active centers

Aspartate aminotransferase (AspAT, EC 2.6.1.1) was bound on CNBr-activated Sepharose and the effects of immobilization on the maximum velocity, biologically active pyridoxal-5'-phosphate (PLP), and transaminationable active centers were studied. By comparing these parameters of soluble and immobilized enzyme the factors decreasing the observed reaction rate upon immobilization were evaluated. Ninety percent of the soluble protein in the coupling mixture was bound to the support. The amount of enzyme-bound PLP of immobilized preparation was 83% of that of the soluble one. The coupling recovery of specific activity was 46%, which was 10%-units lower than that of the transaminationable active centers. This difference depends on the fact that a part of the active centers of immobilized enzyme had lower catalytic rate, due to the enzyme-matrix interactions or internal mass transfer limitations, than the others. The immobilized catalytically active AspAT had 80% of the turnover efficiency of the soluble enzyme. The affinity of the enzyme to its substrates did not significantly change upon immobilization, neither did the pH profile.