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Methyl group transfer from exogenous S-adenosylmethionine on to plasma-membrane phospholipids without cellular uptake
Abstract:
At external concentration of 50 microM, L-methionine was rapidly taken up by hepatocytes, whereas almost no S-adenosylmethionine (SAM) was removed from the incubation medium. SAM did not enter the intracellular water space but equilibrated with a very small pool, which was most likely to be situated on the external side of the plasma membrane. Methyl groups from external L-methionine, but not from external SAM, were incorporated into total and nuclear RNA. A significant incorporation of methyl groups into phospholipids occurred not only with methionine but also with SAM. After subfractionation of hepatocytes it became evident that methyl groups from SAM were mainly incorporated into plasma-membrane phospholipids, and that phospholipid methylation in other cellular compartments resulted from contamination with plasma membrane. The pattern of methylation of the various phospholipid species with SAM as precursor was different from that obtained with L-methionine. In contrast with external L-methionine, external SAM did not enter the intracellular SAM pool. According to these results a transport system for SAM does not exist in rat hepatocytes, although methyl groups from external SAM can be incorporated into plasma-membrane phospholipids from the outside.
Insights
Rat hepatocytes rapidly absorb L-methionine but not S-adenosylmethionine (SAM). External SAM methyl groups are incorporated into phospholipids, particularly in the plasma membrane, but do not enter the cell.
Area of Science:
- Biochemistry
- Cell Biology
- Molecular Biology
Background:
- Cellular uptake and utilization of essential nutrients like L-methionine and S-adenosylmethionine (SAM) are critical for metabolic processes.
- Understanding the transport mechanisms and metabolic fate of these compounds is key to comprehending cellular methylation pathways.
Purpose of the Study:
- To investigate the uptake and utilization of external L-methionine and S-adenosylmethionine (SAM) by isolated rat hepatocytes.
- To determine the intracellular fate of methyl groups derived from L-methionine and SAM, focusing on RNA and phospholipid methylation.
- To elucidate the existence and characteristics of a transport system for SAM in rat hepatocytes.
Main Methods:
- Incubation of isolated rat hepatocytes with L-methionine and S-adenosylmethionine (SAM) at a concentration of 50 microM.
- Measurement of the uptake of L-methionine and SAM from the incubation medium.
- Analysis of methyl group incorporation into total RNA, nuclear RNA, and phospholipids.
- Subfractionation of hepatocytes to localize methyl group incorporation into specific cellular compartments, particularly the plasma membrane.
Main Results:
- Hepatocytes rapidly took up L-methionine, but minimal S-adenosylmethionine (SAM) was removed from the medium.
- External SAM did not enter the intracellular water space but appeared to equilibrate with an extracellular pool near the plasma membrane.
- Methyl groups from L-methionine were incorporated into RNA and phospholipids, while methyl groups from external SAM were incorporated into phospholipids, primarily in the plasma membrane, but not RNA.
- Phospholipid methylation patterns differed between L-methionine and SAM as precursors, and SAM did not enter the intracellular SAM pool.
Conclusions:
- Rat hepatocytes lack a transport system for S-adenosylmethionine (SAM).
- Methyl groups from extracellular SAM can be incorporated into plasma membrane phospholipids, suggesting an external methylation capability.
- The findings highlight distinct metabolic fates and transport limitations for L-methionine and SAM in hepatocytes.