Cotyledons glyoxysomes

Microbodies (97-101) are spherical organelles (0.1-2.0 pm in diameter) bounded by a single membrane. They possess a granular interior and sometimes crystalline protein body. A specialized type of microbody is the glyoxysome (0.5-1.5 pm) containing enzymes ofthe glyoxy-late cycle. Glyoxysomes are found in the endosperm or cotyledons of oily or fatty seeds. 

One class of such organelles is the glyoxysome which is prominent in a wide range of storage tissues, e.g. cotyledons of castor bean, water melon, peanut and cucumber and in pine seeds (Galliard, 1980). The glyoxysome contains all the enzymes of the -oxidation complex, together with the fatty acid... 

Isocitrate lyase has been purified from the glyoxysomes of cucumber (Cucumis sativus) cotyledons. The glycoproteinaceous nature of the enzyme was demonstrated by periodate oxidation studies, and by the in vivo incorporation of 2-amino-2-deoxy-D-[ H]glucose into a species which was precipitable by antibodies to isocitrate lyase. 

Since fats are stored in oil bodies, it is reasonable to expect that the enzyme responsible for their degradation should be closely associated with these structures. During the first 11 days after imbibition a peanut cotyledon may decrease in dry weight from 345 mg to 143 mg, with a concomitant decrease in fat content of 55%. This represents hydrolysis of 9.4 pmoles of triglyceride per cotyledon per day. Even so, less than 1% of the total lipase activity of a peanut cotyledon has been found associated with the oil body and 99% is associated as an acid lipase (pH 4.6) with a particulate fraction [76]. This fraction has been claimed to be mitochondrial, but that is unlikely. The precise location of the acid lipase is still undetermined but it could be associated with the glyoxysomes. 

Fig. 6.12A and B. Variation in glyoxysomal isocitrate lyase ( ) and malate synthetase (o) in (A) isolated peanut cotyledons incubated in darkness and (B) of these enzymes plus catalase ( ) in the glyoxysomal fraction from the scutellum of germinated maize Zea mays). After Longo and Longo, 1970 [85]... 

Fig. 6.13. (A) Diagrammatic representation of the appearance of cucumber seedlings grown under a 12-12 h light-dark cycle. (B) Changes in glyoxysomal and peroxisomal enzyme activities in homogenates of cucumber cotyledons grown as in (A). Enzyme activity glyco-late oxidase ( ), nmol substrate consumed/min/cotyledon isocitrate lyase ( ), 0.1 x nmol substrate consumed/min/cotyledon malate synthetase (a), 0.04 x nmol substrate consumed/ min/cotyledon and catalase (x), 0.2 x units/cotyledon. After Trelease etal., 1971 [139]...

The increase in glyoxysomal enzymatic activities of control cotyledons was paralleled with lipid degradation (1,2),These activities were maximal the first five days of seeds imbibition, afterwards they decreased considerably simultaneously to using up of lipid reserves. Salt delayed optimal activities of studied enzymes, this was paralleled with the decrease in lipidic reserve utilisation (1) NaCl concentration more than 2g/l inhibited paratially enzymes since their maximal activities were reduced in comparaison to control ones. 

BIELIOGRAPHIE (l)Ben Miled D. et A. Cherif-1986- Effet de NaCl sur la degradation des lipides de reserves au cours de la germination des graines de Medicago. Ben Hamida F.,ed,CNRS,Paris,451-453. (2) Tchang F.,Robert 0. et Mazliak P.-1980-Utilisation des reserves lipidiques et formation des glyoxysomes et d etio-plastes dans les cotyledons de Tournesol.Physiol.Veg.,18,117-130. 

Glyoxysomes isolated from sunflower cotyledons metabolized [U- C]llnoleate (1.31 nmol 5.35 MBq ixmor ) to [ C]acetyl-CoA In the presence of ATP, CoASH, and NAD (for principal composition of reaction mixtures see [6]). The percentage of radioactivity detected in acetyl-CoA varied from 44 to 67% after the acetyl-CoA formation had ceased in the reaction mixture. Neither this percentage of radioactivity... 

Using linoleoyl-CoA (1.31 nmol) as substrate of 8-oxidation and following its degradation by HPLC analysis of the reaction mixture, transient accumulation of acyl-CoA intermediates was observed besides accumulation of acetyl-CoA. After 20 min of incubation the sole acyl-CoA detectable in the reaction mixture was acetyl-CoA. The results presented demonstrate that linoleate can be degraded completely to acetyl-CoA in glyoxysomes from sunflower cotyledons. 

In the present study, we isolated and characterized pumpkin cytosolic aconitase cDNA. Fluorescent microscopic observation of the etiolated pumpkin cotyledons using an antibody raised against the aconitase expressed in E. coli clearly showed that no glyoxysomal aconitase exists in etiolated pumpkin cotyledons. 

De Beilis L, Hayashi M, Biagi PP, Hara-Nishimura I, Alpi A. Nishimura M. Immunological analysis of aconitase in pumpkin cotyledons the absence of aconitase in glyoxysomes. Physiol Plant 1994 90 757-762. 

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