Photeolytic activation of a lipolytic enzyme activity in potato leaves

Potato (Solanum tuberosum L.) leaves were shown to contain a lipolytic enzyme activity which is stimulated by treatment with purified trypsin, pronase, and to a lesser degree by chymotrypsin. This protease-stimulated activity was stable over a wide range of pH values. Lipolytic enzyme activity also appeared to be regulated by pH, with a pronounced stimulation at pH 6.0 ± 0.5 and a subsequent inactivation at pH 8.0–9.0. This pH stimulation was slightly by ethylene diamine tetracetic acid (EDTA), and was inhibited by Ca²⁺. Although leupeptin slightly inhibited the pH stimulation, two other protease inhibitors, phenylmethylsulfonyl fluoride (PMSF) and soybean trypsin inhibitor showed no effect. While some of the lipolytic enzyme activitiesn potato leaves (those detected by 1-acyl-2-6-(7-nitro-2,1,3 benzoxadiazol-4-yl) amino]-caproyl] phosphatidylcholine (C₆-NBD-PC) hydrolysis) are stimulated by protease or pH treatment, others (those detected by 4-methylumbelliferyl laurate (4MUL) hydrolysis) are inactivated by them. The possible physiological significance of this apparent proteolytic activation is discussed.