Biosynthesis of Galactolipids in Plants

A Ongun,J B Mudd

doi:10.1016/s0021-9258(18)93578-7

A Ongun, J B Mudd

Open Access

https://doi.org/10.1016/s0021-9258(18)93578-7

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Abstract

Abstract Spinach chloroplasts very actively incorporated radioactivity from UDP-galactose-14C into lipids. Among the products formed, monogalactosyl and digalactosyl diglycerides together with a third compound, tentatively identified as the trigalactosyl diglyceride, contained 95% of the total lipid radioactivity. Chloroplasts from young leaves showed higher enzymic activity than those obtained from mature leaves. Dissociation of the enzymes from the acceptors was accomplished by acetone powder preparation of the chloroplasts. It was shown that diolein could serve as an efficient acceptor for the monogalactosyl diglyceride synthesis but not for the digalactosyl diglyceride synthesis. Monogalactosyl diglyceride, naturally found in chloroplasts, was shown to be the acceptor for the second galactosylation. Incubation of radioactive digalactosyl diglyceride with the acetone powder of spinach chloroplasts resulted in the formation of trigalactosyl diglyceride when cold UDP-galactose was used as the substrate. Evidence indicated that two enzymes were involved in galactolipid synthesis in spinach chloroplasts. The enzyme responsible for the synthesis of monogalactosyl diglyceride was more tightly bound to the membranes than the enzyme synthesizing digalactosyl diglyceride. Particles from etiolated pea leaves as well as other nonchlorophyllous tissues were shown to contain active enzymes that synthesize the galactolipids. A light-induced increase in the enzyme activity was observed in pea leaves.

Highlights

Lipids by Particles from Different Sources-In light of the results reported in Table III, it was of interest to determine whether particles from nonchlorophyllous tissues would synthesize galactolipids
The spinach chloroplasts used by Ncufeld and Hall [17] incorporat,ed only 307; of the added radioactivity into lipids, whereas in our case we obtained 100(~;~ incorlroration with either the chloroplasts of greenhouse-grown spinach plants or of those obtained from local mark&s
In the course of our experiments it became apparent that spinach and pea leaves as well as other plant tissues that we uved contained soluble enzymes that could break down IJDP-galactose, making it unsuitable as the galactosyl donor

Summary

Methods

Isolation of PnrticlesChloroplasts were isolated from 20 to 25 g of deveined leaves which were chilled at 4” for 1 t,o 2 hours prior to grinding. Leaves were ground in a mortar with acidwashed sand in 0.5 M sucrose solution containing 0.01 M phosphate buffer at pH 7.4. The homogenate was filtered through four layers of cheesecloth and the filt’rate was centrifuged at 200 X g for 2 min. 1,000 x g for 7 min to obtain the chloroplasts. The supernatant was decanted and the side walls of the tube were wiped clean before resuspension of the pellet.

Results

Discussion

Conclusion