Role of methionine as a methyl group donor in cattle Role of methionine as a methyl group donor in cattle

, Betaine, Choline,


Introduction
Methionine is an essential amino acid often identified as limiting for growing ruminants.Methionine functions as a precursor for protein synthesis, and a deficiency of this amino acid can cause inefficient use of dietary protein for lean muscle (protein) deposition.However, methionine has many other functions in the body, including methyl group donation and use for cysteine and polyamine biosynthesis.
More than half the methionine requirement of rats can be replaced by cysteine.However, recent research with cattle has indicated that cysteine does not effectively spare methionine.The lack of response to cysteine may have been due to methyl groups being limiting, such that methionine was needed as a methyl group donor.Therefore, our objective was to evaluate the sparing of methionine by alternative sources of methyl groups (betaine or choline).

Experimental Procedures
Experiment 1. Five ruminally cannulated Holstein steers (343 lb initial BW) were maintained in metabolism crates to facilitate total collection of feces and urine.Steers were limit fed (5.3 lb/day, dry basis) a diet based on soybean hulls (84% soybean hulls, 7% wheat straw, 3% molasses, 5% minerals/vitamins, and 0.5% urea).This diet was formulated to contain a low amount of undegradable intake protein so that only limited amounts of dietary amino acids were available postruminally.T o increase energy supply without increasing ruminal microbial growth, steers received a continuous infusion of volatile fatty acids (180 g acetate, 180 g propionate, and 45 g butyrate/day) into the rumen and a continuous infusion of glucose (300 g/day) into the abomasum.Also, an amino acid mixture containing 150 g L-glutamate; 50 g glycine; 20 g L-valine; 30 g L-leucine; 20 g Lisoleucine; 40 g L-lysine-HCl (feed grade, 79.8%); 10 g L-histidine-HCl-H 2 O (74%); 20 g L-arginine; 20 g L-threonine (feed grade, 98%); 35 g L-phenylalanine; 7 g L-tryptophan (feed grade, 98%); and 2 g L-methionine per day was infused continuously into the abomasum to ensure that nonsulfur amino acids did not limit tissue protein synthesis.The abomasal infusions were made by placing flexible tubing (inside diameter: 1/16 in.) through the rumen cannula and the reticulo-omasal orifice.
A 5 × 5 Latin square design was used with periods of 7 days.This allowed for a 2-day adaptation to the abomasal infusions and 5 days for total collection of feces and urine.Treatments were abomasal supplementation with 1) water (control), 2) 2 g/day additional Lmethionine, 3) 1.7 g/day L-cysteine, 4) 1.6 g/day betaine, and 5) 1.7 g/day L-cysteine + 1.6 g/day betaine.The L-cysteine and betaine were provided in amounts that were equimolar to the L-methionine supplement.Only four observations were obtained for the 1.7 g/day Lcysteine + 1.6 g/day betaine treatment because of an infusion problem in the last period.
Experiment 2. Five ruminally cannulated Holstein steers (348 lb initial BW) were used in a design similar to Exp. 1 and were housed in similar conditions.They were limit-fed 5.5 lb/day, dry basis.
Treatments were abomasal supplementation with 1) water (control), 2) 2 g/day additional Lmethionine, 3) 8 g/day betaine, 4) 16 g/day betaine, and 5) 8 g/day choline.The betaine was provided in amounts that were 5 and 10 times the amount in Exp. 1 in order to test the efficiency of betaine as a methyl group donor.Choline was infused to examine its role as an alternative methyl group donor.

Results and Discussion
Experiment 1. Increases in retained nitrogen were due to decreases in urinary nitrogen excretion (Table 1).
Nitrogen retention increased in response to supplementation with 2 g/day methionine (P<.05), but responses to equimolar amounts of cysteine and betaine alone or in combination were less dramatic.The response to methionine verifies the sulfur amino acid-deficient conditions intentially created by our model, whereas the low response to cysteine supplementation may indicate that the response to additional methionine supplementation is not due to conversion of methionine to cysteine.However, both cysteine and betaine tended (P<.16) to increase nitrogen retention relative to the control treatment.Because of the insignificant response to methyl groups (betaine), the sparing of methionine by methyl donors could not be demonstrated.However, replacement of methionine by betaine appeared to be relatively inefficient (the response to betaine was 20% as large as that to methionine).Therefore, we hypothesized that more betaine may be required to yield responses similar to that observed for methionine.
Experiment 2. As in Exp. 1, observed increases in retained nitrogen resulted from decreases in urinary nitrogen excretion (Table 2).Also, large increases in retained nitrogen occurred for steers supplemented with 2 g/day methionine (P<.05).However, betaine infused at levels that were 5 and 10 times higher than that supplied in Exp. 1 only tended (P<.22) to increase nitrogen retention.Nitrogen balance responses were only 23% (for 8 g/day betaine) and 20% (for 16 g/day betaine) as large as that observed for methionine.These responses were similar to that for the lower level of betaine in Exp. 1. Choline also failed to improve nitrogen balance.