Enzymes of assimilatory sulfate reduction in leaves of <i>Pisum sativum</i>: Activity changes during ontogeny and in vivo regulation by H₂S and cyst(e)ine

Abstract

Enzyme activities of assimilatory sulfate reduction were measured in leaves of Pisum sativum L., cv. Vatters Frühbusch, during their ontogenetic development, and during treatment with H 2 S and cyst(e)ine. Ribulose bisphosphate (RuBP) carboxylase (EC 4.1.1.39) and ferredoxin‐dependent nitrite reductase (Fd‐NiR, EC 1.7.7.1) were measured for comparison. In etiolated pea leaves, ATP‐sulfurylase (ATPase, EC 2.7.7.4), adenosine 5′‐phosphosulfate sulfotransferase (APSSTase), ferredoxin‐dependent sulfite reductase (Fd‐SiR, EC 1.8.7.1) and O‐acetyl‐L‐serine sulfhydrylase (OASSase, EC 4.2.99.8) activities were measured in appreciable rates, while neither RuBP carboxylase nor Fd‐NiR activities could be detected. During the first 2–7 days after transfer into the light all enzyme activities increased. After reaching maximal activities, ATPase, APSSTase, and Fd‐SiR activities decreased in all leaves to low or indetectable levels during the following 3–6 days. RuBP carboxylase, Fd‐NiR and OASSase, on the other hand, decreased slowly and were still at high levels of activity at the end of the experiment. Fumigation of pea plants with 1.5 μl l −1 H 2 S delayed the initial increase and the subsequent decrease of ATPase activity by 1–3 days. APSSTase activity decreased for 1–2 days, increased rapidly during the next 4–6 days and retained a high level of activity until the end of the experiment as did Fd‐SiR. One to two days after the beginning of fumigation the leaves started to accumulate high amounts of cyst(e)ine. When pea plants with excised roots were placed on a nutrient solution containing cyst(e)ine, APSSTase activity decreased more on 0.2 and 0.5 m M than on 1.0 m M. Fd‐SiR activity was only slightly decreased on 1.0 m M cyst(e)ine. Neither Fd‐NiR nor RuBP carboxylase activities were affected.