Graduate Thesis Or Dissertation
 

Characterization of different forms of lysophospholipase in barley

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  • During barley germination, α-amylase and other hydrolases are synthesized in the aleurone layer and secreted into the starchy endosperm. As starch is degraded by amylases, lysophospholipids are released from inclusion complexes with the starch and undergo hydrolysis by a lysophospholipase (LPL) activity. Studies with embryo-free half seeds and isolated aleurone layers from barley (Hordeum vulgare L. cv Himalaya) show that LPL activity appears in the aleurone layer of imbibed half-seeds. Secretion, on the other hand, requires the continued presence of gibberellic acid (GA₃) and commences after a 10 to 14 hour lag period. Ca²⁺ alone has very little effect on the level of LPL activity in the aleurone layer or that secreted into the surrounding medium. However, 50 mM Ca²⁺ together with GA₃ dramatically increases the total activity. Density labeling experiments with deuterium oxide indicates that GA₃-stimulated LPL is synthesized de novo. Two forms of LPL activity are distinguished by ion exchange chromatography on carboxymethyl cellulose. A predominantly basic activity is observed in the starchy endosperm of germinated barley and the incubation medium of GA₃-stimulated half-seeds. In contrast a predominantly acidic activity appears in the incubation medium of GA₃-treated aleurone layers. The predominantly acidic activity from the medium of GA₃-stimulated aleurone layers is converted to the basic form in the presence of starchy endosperm, amylase-digests of barley starch or EDTA. The factor from starch is thermally stable. Divalent metal ions do not shift the basic activity to the acidic form. On gel filtration chromatography the acidic LPL elutes at a high molecular weight position (M[subscript r] ~ 160000) distinct from the basic activity (M[subscript r] 40000). The elution volume of the acidic activity treated with EDTA shifts to that of the basic activity. Anti-serum prepared against a purified basic LPL generates similar enzyme inactivation profiles against extracts of both the predominantly basic and acidic LPLs.
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