Isoenzymes of Glutamine Synthetase in Roots of Pea (Pisum sativum L. cv Little Marvel) and Alfalfa (Medicago media Pers. cv Saranac)

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Abstract
Cell organelles have been isolated from protoplast lysates and total homogenates obtained from root tips of Pisum sativum L. (cv Little Marvel) and Medicago media Pers. (cv. Saranac) grown in hydroponics with nitrate nutrient solutions. Density-gradient and differential centrifugation procedures have been used to prepare mitochondria- and plastid-enriched fractions in which glutamine synthetase (GS) activity was estimated. Even when purified protoplasts were gently ruptured, significant breakage of plastids occurred during preparation as shown by the high proportion of nitrite reductase recovered in the soluble fraction, depending on the source of plant material and the GS assay in which glutamine production was monitored by high performance liquid chromatography was devised to estimate the physiological significance of the transferase and the semibiosynthetic assays currently used for activity measurements. When compared with the true and semibiosynthetic assays, the transferase assay for GS appeared to underestimate the root plastid enzyme. Root plastid GS was partially purified by ion-exchange chromatography, and results show that the isoenzyme found in root plastids is different from chloroplastic or cytosolic GS.