Alkaline phosphodiesterase I (5'-nucleotide phosphodiesterase, EC 3.1.
4.1) in cultured cells of Mentha arvensis, was purified about 75-fold
by ammonium sulphate fractionation and three chromatographic steps. Th
e optimum pH of the enzyme was 9.5. Its M(r) was estimated by gel filt
ration to be cn 105000. The enzyme was strongly inhibited by SH reagen
ts and HgCl2. It did not require divalent cations such as Mg2+ or Ca2. It hydrolysed thymidine 5'-p-nitrophenylphosphate but did not act on
DNA or RNA. These properties, such as divalent cation requirement and
substrate specificity, were different from those of phosphodiesterase
I obtained from carrot.