H3 TLysine

Hj Hj Hj H

FiSur,a.S2 L-lpineis L-lysine (2.6-diaminohexanoic acid alpha, epsilon-diaminocaproic acid one-letter jn ««ntial wnino .u-td code K: molecular weight 146) is an essential amino acid with 19.2% nitrogen.

Abbreviation

Lys L-lysine

Nutritional summary

Function; The essential amino acid L-lysine (Lys) is needed for the synthesis of peptides and proteins, and as a precursor of'camitine. I :se as an energy fuel requires adequate supplies of thiamin, riboflavin, niacin, vitamin B6, pantothenate, lipoate. ubiquinone, iron, and magnesium.

Food sources: Adequate amounts are consumed when total protein intakes meet recommendations. Dietary supplements containing crystalline Lys are commercially available.

Requirements; Estimates of daily Lys requirements vary, but appear to he near 30mg/kg in healthy adults (El-Khoury el id., 2000).

Deficiency: Prolonged lack of Lys. as of all essential amino acids or a lack of protein, causes growth failure, loss of muscle mass and organ damage. Excessive intake: Very high intake of protein and mixed amino acids (mote than three times the RDA or 2.4 g kg) is thought to increase the risk of renal glomerular sclerosis and accelerate osteoporosis. There have been anecdotal reports of symptoms alike to eosinophil^- myalgia syndrome and other severe illnesses follow ing use of some synthetic Lys preparations.

Dietary sources

AH protein-containing foods provide some Lys. In addition, significant amounts of hydroxyllysine arc consumed with meats. Foods relatively enriched in Lys (as a percentage of total protein) include pork (9.0%), beef (8.3%). chicken (8,0%). cow's milk (7.9%), eggs (7.2%), and soy (6.5%). Grains contain a much smaller percentage, as seen with rice (3.6%). wheat (2.7%), and corn (2.8%).

Typical Lys intake of American adults is 4.5 g day: vegetarians get considerably less (Sschan etai, IW7).

Heat treatment (baking, broiling, grilling) can chemically alter Lys and induce the formation of so-called Mai I lard products, conjugates of Lys and sugars (Dworschak. 1980),

Digestion and absorption

Various enzymes from stomach, pancreas, and intestine hydrolyze food proteins. The hydrogen ion peptide (»transporter I (SLCI5AI. PepT 11 and to a much lesser extent the hydrogen ion peptide cotransporter 2 (SLCI5A2, PepT2) mediate uptake of di-and tripeptides w ith broad specificity.

Free Lys can enter intestinal cells via the y~. sodium-independent cationic amino acid transporters 1 (CAT-I, SLC7A1) and 2 (CAT-2. SLC7A2). The sodium-dependent system B" " seems to constitute only a minor transport route, possibly in the dislal diitn peptides

Intestinal lumen diitn peptides

Intestinal lumen

Enterocyte

Brush border membrane

Figure 8.53 Intestinal absorption of L-lysinc

Lys neutral "a V Na*

fy'LATl' 4F2

Enterocyte

Capillary lumen

Brush border membrane

Figure 8.53 Intestinal absorption of L-lysinc

6a sol at era I membrane

Capillary endothelium small intestine. Uptake can also proceed through the rBAT (SLC3A1 (-anchored amino acid transporter BATl b" (SLC3AI) at the brush border membrane, which can shuttle Lys into the enterocyte in exchange lor a neutral amino acid plus a sodium ion (Chairoungdua et al., 1999).

Export of Lys towards the pericapillary space uses two transporters that are anchored to the basolatcral membrane by glycoprotein 4F2 (SLC3A2). Both y+LAl 1 (SLC7A7) and v' LAT2 (SLC7A6) move Lys into the basolatcral space in exchange for a neutral amino acid and a sodium ion.

Note: The conjugates Lys forms with other amino acids or sugars during heating of foods in a browning (Mai Hard) reaction are not cleaved or absorbed in the small intestine. Examples of such heat-generated compounds include lysinoalanine, fructoselysine, and N epsilon-carboxymethyl lysine.

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