SOYBEAN SEED ACID-PHOSPHATASES - UNUSUAL OPTIMUM TEMPERATURE AND THERMAL-STABILITY STUDIES

In contrast to other acid phosphatases, four cytoplasmic isoforms (AP1 , AP2, AP3A and APSB) purified from mature soybean seeds presented hig h activities at temperatures above 80 degrees C, when p-nitrophenylpho sphate (p-NPP) was utilized, as substrate. However, with tyrosine phos phate and inorganic pyrophosphate as substrates, maximum activities we re observed at temperature of 60 degrees C during 10 min reaction, In the absence of substrate, enzymes lost only 20% activity after 60 min at 60 degrees C; the isoforms AP3A and AP3B retained 30% of activity a t 70 degrees C after 60 mill and all the isoforms were inactivated at 80 degrees C, after 5 min, Thermal. inactivation studies indicated tha t the soybean enzymes showed different temperature dependences in rela tion to most plant acid phosphatases, A best protective effect was obs erved when the isoforms were preincubated, at 70 degrees C, with phosp hate (10 mM) and p-nitrophenol (10 mM) which. indicates that the enzym e inactivation was prevented only in the presence of both reaction pro ducts. (C) 1998 Academic Press.