Stability of vitamin B-6-dependent aminotransferase activity in frozen packed erythrocytes is dependent on storage temperature

Pyridoxal 5 ' -phosphate (PLP) stimulation of erythrocyte alanine and aspar tate aminotransferase (EALT, EAST) activities is a frequently used function al measure of vitamin B-6 status. Stability of enzyme activities and activi ty coefficients (AC, stimulated divided by unstimulated) was assessed in pa cked erythrocytes frozen at -20, -80 degreesC and under liquid nitrogen (-1 96 degreesC), Activities of EALT and EAST, with and without added PLP, were determined in fresh erythrocytes (d 0) and frozen samples on d 1, 7, 14, 2 8, 58 and 84. In -20 degreesC samples, EALT basal activity decreased 17 and 22% (P less than or equal to 0.05 for both) by d 58 and 84, respectively, and EAST basal activity decreased 40% (P less than or equal to 0.05) by d 5 8, In -80 and -196 degreesC samples, EALT and EAST basal activities did not change significantly. Activity coefficients did not differ significantly f rom d 0 at any storage temperature, but EAST-AC increased 9-19% (nonsignifi cant) in samples stored at -20 and -80 degreesC for 7 to 84 d, Additionally , EAST-AC was significantly higher in -20 than -80 and -196 degreesC sample s on d 1 and 58, respectively. Erythrocytes may be frozen for 28 d at -20 d egreesC and 84 d at -80 degreesC before analysis for EALT; for EAST, activi ty should be measured on fresh erythrocytes.