Table 1. Precision Data VitaminB6MeasuredIn ... - Clinical Chemistry

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pyridoxal 5'-phosphate with a pCO2 elec- trode. Anal C/tern 53, 512-515. (1981). Charles. Plese. William Fox. Kathy Williams. Biomedical. Reference. Labs, Inc.

cause a “rise” (an increase in absorbance over a threshold indicative of a sample peak) in the AST blank channel. The concentration of metronidasole in the primer is critical; too little and the blank channel shows no rise; too much and the AST activity in the test channel isapparently so depressed by the drug that the trigger level (threshold) is not reached. The minimum AST activity to cause a rise is 40 U/L, while the minimum concentration of metronidazole to cause a rise in the blank channel is 15 mg/L, equivalent to AST activity of about -40 U/L. Consequently, we recommend that the primer be prepared as follows: Calibration reference serum: 10 mL Phasing dye (Technicon): 0.2 mL Metronidazole (5 g/L): 40 pL This formula provides a metronidasole concentration of 20 mg/L, equivalent to AST activity of about -50 U/L, sufficient to effect a rise in the blank channel and yet not excessively depress the test channel activity. For example, if AST activity of the calibration reference serum is 100 UIL, minus the apparent metronidazole activity of 50 U/L, then the effective activity of the primer will be 50 U/L-sufficient to cause a rise,and with both test and blank channels having a safe margin for error. A minor problem with assay nomenclature on printout and video display should be corrected when the next SMA U disc revision is made. This medification has been in routine use in our laboratory since April 1982. Negative and factitiously low AST values have been eliminated.

Bevan

D. Hokin

Dept. of Pathol. Middlernore Hosp. Auckland 6 New Zealand

VitaminB6MeasuredIn Plasma with a C02-Selectlve Electrode To the Editor: Hitherto, analysis for the biologically active form of vitamin B6 (pyridoxal phosphate) has required the use of radioactive material or complex instrumentation. We describe here a rapid method for determination of the active form of vitamin B6 in plasma by use of a C02-selective electrode. The methed depends on conversion of the apoenzyme of tyrosine decarboxylase (EC 4.1.1.25) into its active form by pyridoxal phosphate. The enzyme then catalyzes the breakdown of tyrosine and C02, the rate of which is monitored by the CO2 electrode.

After equilibrating the CO2 electrode with the solution for about 5 mm, we started the reaction by injecting 200 jL of tyrosine under continuous stirring. The rate of reaction was evaluated in millivolts per minute. After the coupling reaction, total analysis time is about 10 mm per specimen. A typical standard curve is shown for extracted pyridoxal phosphate in Figure 1. The precision data obtained from the

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