Nutritional summary

Function: The nonessential amino acid L-serine (Ser) is needed for the synthesis of proteins, selenoeysteine. and 3-dehydro-D-sphinganine, and is a potential precursor of glycine. L-cysteine, and L-alanine. Its use as an energy fuel depends on adequate availability of thiamin, riboflavin, niacin, vitamin B6. folate, pantothenate, lipoate. ubiquinone, iron, and magnesium.

Food sources: Adequate amounts are consumed w hen tola) protein intakes meet recommendations. Exposure to high heat causes losses due to oxidation. Requirements: There are no specific requirements as long as total protein intakes art-adequate.

Deficienty: Since lack of Ser occurs only in severe protein deficiency, the symptoms arc those of severe starvation.

Excessive intake: Very high intake of protein and mixed amino acids (more than three times the RDA or 2.4 g kg) is thought Hi increase the risk of renal glomerular sclerosis and accelerate osteoporosis. The consequences of very high intakes of Ser have not been adequately evaluated.

Glucose

COOH I

CH-OH I

NAD NADH

D - 3- P hosp hogly cerate dehydrogenase

COOH

CHs O

3-Phosphoglycerate

3- Phosphoglydroxypyruvate

3-Phosphoserine aminotransferase (PLP)

u-keloglutarale

L-glutamate

COOH I

Chfe

3-Phosphoserine

D-3-Phosphoserine phosphatase <Mg">

5,10 methylen COOH THF THF

COOH

Serine hydroxy-methyttransferase

Glycine

Figur« 8.24 Endogenous synthesis of L-serine

OH L-Sen ne

Endogenous sources

Ser is released with the breakdown of proteins, and is newly synthesized from the glycolysis intermediate 3-phosphoglyeerate. from glycine, and indirectly from hydroxypro-linc in the kidney (Lowry a at,. 1985).

Synthesis from glycolysis intermediates: Especially when protein intakes are low, Ser synthesis can originate from 3-phosphoglyeerate. Phosphoglyccrate dehydrogenase

(ECI.1.1.95), phosplioserine aminotransferase (Hi'2.6.1.52. PLP-dcpcndent), and phosphoserine phosphatase (EC3.1.3.3, requires magnesium) catalyze this sequence of reactions.

Synthesis fwm glycine: Glycine hydroxy me thy Itrans fera se (EC2.I.2.I) mediaies the one-carbon transfer from 5,1Q-methylenetetrahydrofolate to glycine. Under most circumstances, however, this reaction runs in the reverse direction. Synthesis ofD-serwe: A specific enzyme in brain, serine racemase (no EC number), converts L-serine into its enantiomer D-serine (Wolosker el ai. 1999),

Dietary sources

Most dietary proteins contain about 4-5"<i Ser. Moderately enriched sources are eggs (7.4%). beans (5.5%). milk (5.4%). and rice (5.3%); chicken meat contains a slightly smaller percentage (3.4%), I leat treatment of foods decreases the amount of bioavaliable Ser(Dworschak, 19*1)).

Many foods also contain some phospho-L-scrine: minor amounts of the enantiomer l)-serine also may be present in some.

Digestion and absorption

Various enzymes from stomach, pancreas, and intestinal wall break down Ser-containing proteins (see amino acid overv icw chapter), none of them with particular preference for Ser residues.

ditri peptides

Ser neutral amino i ditri peptides

-. Na neulial amino a

Ser neutral amino i

Dnzr"

Dnzr"

Intestinal lumen

Capillary lumen

Brush border membrane

Basóla teral membrane

Capillary endothelium

Figure 8 JÍ Intestinal absorption of serinif

Ser-containing di- andtripeptidescan also be taken up via the hydrogen ion peptide eotransporter I ISLC15AI. PepT 11 and. to a much lesser extent, via hydrogen ion peptide eotransporter 2 (SLC15A2. PcpT2). Measurements in rabbit small intestine indicate that about half of Ser uptake across the brush border membrane is mediated by sodium-am ¡no acid eotransport system 13". the other half by system ASC (Munck and Munck. 1990; Avissar et al.. 2001 ). The sodium-independent rBAT (SLC3AII glyeoprotein-anchored transporter BATI b" ' (SLC7A9) uses Ser in most situations as a counter molecule in exchange for the transport of other neutral amino acids and usually effects net Ser transport into the lumen.

Some of the Ser taken up is used for the enterocyte's own protein synthesis; significant amounts are converted to glycine.

Export of Ser across the basolatcral membrane uses mainly the sodium-ami no acid eotransport systems A (ATA2) and ASC( ASCTI ). LAT2<SLC7AK). a 41'2-glyeoprotein (SLÇ3A2}-anchored member of the system L family, can transport Scr in either direction across the basolatcral membrane in exchange for another neutral amino acid. Depending on the difference between intracellular and interstitial Ser concentrations LAT2 and the sodium-independent transporter asc can also effect net export.

Starvation increases expression of the transport systems A and L. whereas ASC-mediated uptake will nol be affected (Muniz et al.. 1993).

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