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Glycosyltransferases in the Golgi membranes of onion stem
Abstract:
Cell fractions consisting largely of Golgi membranes were prepared from the meristematic region of the onion. Several enzyme activities were found to be localized in these fractions: inosine diphosphatase, galactosyltransferases and glucosyltransferases. The fractions catalysed the transfer of [(14)C]galactose from UDP-galactose to endogenous and cell-sap acceptors, to N-acetylglucosamine and to ovalbumin. In the presence of bovine alpha-lactalbumin, transfer to glucose (lactose synthesis) was catalysed. [(14)C]Glucose was transferred from UDP-glucose to endogenous and cell-sap acceptors, to cellobiose and to fructose (sucrose synthesis). All these activities were latent, being potentiated by detergents (Triton X-100 or sodium deoxycholate). The characteristics of some of these enzyme activities are described and their biological significance is discussed.
Insights
This study isolated Golgi membranes from onion cells, revealing key enzymes like galactosyltransferases and glucosyltransferases involved in synthesizing complex carbohydrates and lactose.
Area of Science:
- Plant Cell Biology
- Biochemistry
- Membrane Biology
Background:
- Golgi apparatus plays a crucial role in modifying and transporting proteins and lipids.
- Understanding enzyme localization within the Golgi is vital for deciphering cellular metabolic pathways.
Purpose of the Study:
- To isolate and characterize Golgi membrane fractions from onion meristematic cells.
- To identify and analyze the enzymatic activities present in these fractions, focusing on glycosyltransferases.
- To investigate the potential roles of these enzymes in carbohydrate synthesis.
Main Methods:
- Preparation of cell fractions enriched in Golgi membranes from onion (Allium cepa) meristematic tissue.
- Assay of enzyme activities including inosine diphosphatase, galactosyltransferases, and glucosyltransferases.
- Use of radiolabeled substrates (UDP-galactose, UDP-glucose) and various acceptors to measure enzyme catalysis.
- Investigation of enzyme latency and activation by detergents (Triton X-100, sodium deoxycholate).
Main Results:
- Isolated fractions showed significant inosine diphosphatase, galactosyltransferase, and glucosyltransferase activities.
- Galactosyltransferases transferred [(14)C]galactose to various acceptors, including N-acetylglucosamine and ovalbumin.
- Lactose and sucrose synthesis were observed, catalyzed by specific transferase activities in the presence of appropriate substrates and cofactors (bovine alpha-lactalbumin for lactose).
- All identified enzyme activities were latent and could be potentiated by detergents.
Conclusions:
- Onion Golgi membranes harbor essential glycosyltransferases involved in the synthesis of complex carbohydrates and potentially disaccharides like lactose and sucrose.
- The latent nature of these enzyme activities suggests regulatory mechanisms within the Golgi lumen or membrane.
- These findings contribute to understanding plant cell wall biosynthesis and carbohydrate metabolism at the subcellular level.