Louis, MO, USA), ACE (from rabbit lung), hippuryl-L-histidyl-L-leucine (HHL), hydrindantin
, and lithium hydroxide were purchased from Sigma (USA).
Ascorbic acid addition into ninhydrin reagent prevents oxidation of hydrindantin
. Values for absorbance of ACC solutions were found in the rage 0.015 to 0.3 mmol/1 at 570 nm after the reaction was highly correlated with ACC concentrations ([R.sup.2] = 0.999) and resulted in linear calibration curves by standard ninhydrin assay.
The ninhydrin reagent was prepared by mixing 18.0 g of ninhydrin and 0.7 g of hydrindantin
with a reagent containing (per liter) 767 mL of dimethyl sulfoxide, 225 mL of water, 7.0 g of lithium acetate, and 1 mL of concentrated acetic acid; the reagent was kept at room temperature for 4 h.