Urea and homocysteine in the blood serum of pigs fed diets supplemented with betainę and an enhanced level of B group vitamins *

The aim of the study was to determine the effects of betainę and increased supplementation of vitamins B 6 , B ] 2 and folie acid in pig diets on blood urea and homocysteine levels. The experiment was conducted on 64 growing-fattening pigs. The parameters under study did not differ significantly among the groups. However, in animals fed on betainę non-supplemented diets, the urea level tended to be higher, while homocysteine was lower when diets contained higher levels of B vitamins. The urea level was uniform in pigs fed diets supplemented with betainę, and homocysteine varied to a very smali extent. KRY WORDS: fatteners, betainę, vitamin B 6 , vitamin B p , folie acid, urea, homocysteine


INTRODUCTION
Methionine is an essential amino acid and is not synthesized de novo in mammalian tissues.However, when diets are deficient in methionine, its synthesis in the liver is possible from homocysteine with betainę (trimethylglycine) being the donor of methyl groups.Betaine-homocysteine methyltransferase (BHMT) or methionine synthase (in the presence of vitamin B ]2 ) are involved in the process, which may be inhibited by a Iow level of betainę (Finkelstein et al., 1971;Remus and Virtanen, 1996).According to McDevitt et al. (2000), betainę cannot fully compensate methionine deficiency in animal diets.When there is an excess of methionine, homocysteine can be catabolised by cystathionine P-synthase to cysteine in the presence of vitamin B fi .Besides vitamins B 6 and B 12 folie acid also takes part in metabolizing homocysteine to methionine (Kłosiewicz-Latoszek and Ostrowska, 2000).Some data concerning the use of blood urea levels as an indicator of protein or amino acid was reported by Swiątkiewicz (2003).
The aim of the study was to determine whether there are interactive effects of supplementary betainę and B vitamins on the metabolism of sulphur amino acids in pigs fed diets differing in methionine content in the growing period and with the same methionine content in the finishing phase.Blood levels of urea and homocysteine were used as indicators.

MATERIAŁ AND METHODS
The experiment was carried out on 64 fatteners (crossbreds of Polish Landrace x Polish Large White x Duroc x Hampshire) divided into 8 groups of 4 barrows and 4 gilts.The diets fed during the growing and finishing periods contained barley, wheat, soyabean oilmeal, soyax (fuli fat soyabean), Rovimix (a vitaminmineral supplement), common salt and dicalcium phosphate.The variable components were betainę (0 or 1.25 g/kg), vitamins B (folie acid, B ft and B ]2 ), and DL-methionine in the growing period (Table 1).The diets fed during the growing period (up to 75 kg body weight) had an energy value of 13.05 MJ ME, contained 164 g protein and 10.5 g lysine (per kg), while the proportion of methionine + cystine to lysine was 57 (group I, II, V and VI) or 64 (groups III, IV, VII and VIII).The respective values for diets fed in the finishing period (up to 105 kg body weight) were 12.95 MJ ME, 140 g protein, 8.30 g lysine, and 63 methionine + cystine as related to lysine.Threonine and tryptophan levels in relation to lysine were 64 and 21, respectively, in both periods.The animals were fed individually according to Polish standards (1993).The diets were given in dry form, water was supplied ad libitum.
At slaughter, blood samples were taken from all pigs and urea was determined in blood serum using Vitros eąuipment; homocysteine was analysed by HPLC as described by Ubbink et al. (1991).The results were subjected to one-way analysis of variance using the SPSS 10.1 program.

RESULTS AND DISCUSSION
The parameters under study did not differ significantly among the groups (Table 2).However, within the groups not supplemented with betainę, a tendency was observed towards higher urea and lower homocysteine levels in animals fed diets with higher contents of vitamins B (group II vs I and IV vs III).In the betaine-supplemented groups, the urea concentration was very uniform and the homocysteine concentration also varied to only a smali extent.Higher supplementation of vitamins B may promote the metabolism of homocysteine to methionine or to cysteine.Both pathways can decrease the homocysteine level in blood serum while the non-utilized pool of amino acids may increase the urea content in blood.Betainę added to a diet with a high level of vitamin B )2 or folie acid becomes a more efficient methyl group donor, and due to this, methionine can be better utilized for protein synthesis.Saunderson and MacKinlay (1990) pointed out that a lower methionine content in the diet can cause an increase in BHMT and stimulate remethylation of homocysteine.The opposite opinion was presented by Emmert et al. (1998), who pointed out that deficiency in methionine does not cause increased activity of BHMT, however, betainę supplementation increased its activity in the liver.The results of these two studies were not, however, univocal.It seems that the methionine level in the experimental diet met the reąuirement for this amino acid and supplementation with betainę and vitamins B did not markedly influence the examined parameters in blood.

CONCLUSIONS
An increased level of B vitamins can influence the metabolism of sulphur amino acids and decrease the homocysteine level in blood serum.Urea and homocysteine levels, as parameters related to sulphur amino acid metabolism, seem to depend on yitamins B levels and betainę supplementation.