Chloroplasts are the Subcellular Location of Both Soluble and Membrane-associated Homoserine Kinases in Pea (Pisum sativum L.) Leaves

Abstract

Summary Pea leaf homoserine kinase was associated with intact chloroplasts, but not with naked thylakoids (broken chloroplasts), on sucrose density gradients, however, when isolated intact chloroplasts were lysed in the presence of a protective media, both soluble and thylakoid-associated activities were found. The thylakoid-associated homoserine kinase activity exhibited apparent Km values of 1.2 and 4.7 mM for homoserine and ATP, respectively. The inhibition of the thylakoid-associated homoserine kinase activity by added amino acids was similar to that of the soluble pea leaf homoserine kinase (Thoen et al., Plant Sci. Let. 13, 103-112, 1978), with the exception of S-adenosylmethionine, which when present at 7.5 mM, inhibits the soluble enzyme by 92%, but only inhibited the thylakoid-associated enzyme by 57%. The possible relationship between the soluble and thylakoid forms of the enzyme is discussed.