Graduate Thesis Or Dissertation
 

Lysophospholipase of germinating barley seeds

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  • The amylose component of cereal starch contains lysophospholipid, predominantly lysophosphatidylcholine (LPC) as inclusion complexes. As starch undergoes degradation by amylases during barley germination, lysophospholipase (LPL) an enzyme which degrades lysophospholipids appears in the starchy endosperms and becomes associated with the starch fraction. The enzyme has been reported to have a pH optimum at 8 with an inflection near 5. A study with two varieties of Glacier barley, one with a high amylose content, was conducted to assess the influence of starch com position on its mobilization and on other factors during germination. It was found that there is essentially no difference between the two varieties in the rate at which either total starch or amylose is degraded during germination. Included lipid also has no influence on the rate of amylose degradation. Both LPL activities (pH 8 and 5) increase markedly at day 5 of germination. The proportion of the alkaline activity associated with the starch is greater than for the acidic activity. However, there is no correlation between amylose content or amylose included lipid and starch-bound LPL activity. LPL activity has been purified and characterized from a 4 day, unkilned brewing barley variety Advance. It displays the properties of a basic, lipophilic glycoprotein with a pH optimum at 8. The most active glycoprotein (LPL I) has Km 30 μM, Vmax 200 μmol/min/mg protein against LPC-1-palmitoyl as substrate. It has molecular weight 41,000; pI 8.8 and contains 12% carbohydrate on the basis of molecular weight reduction after chemical deglycosylation. Two other unresolved glycoproteins (LPL II) have Km 30 μM, Vmax 50 μmol/min/mg protein and molecular weights 41,000 and 40,000. They also have pI 8.8 and the same molecular weight polypeptide after deglycosylation as deglycosylated LPL I. No phospholipase A, lipase or transacylase activity is shown by the LPLs. Their substrate specificity coincides with the most abundant barley starch lipids. Mouse antibodies raised against the LPLs have been used to measure LPL in whole endosperms. Although enzyme activity does not increase markedly until day 4 of germination, immunologically active enzyme increases at day 3. The presence of an enzymatically inactive polypeptide in the aleurone, presumably a pre-LPL which is undergoing processing e.g. by glycosylation, is consistent with the observation.
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