An important question in the sulphur amino acid nutrition of animals is the extent to which cystine can meet the methionine + cystine requirements. Because methionine can be converted into cystine but there is no conver sion of cystine into methionine, it has been assumed that methionine can fulfil the need for both methionine and cystine but that cystine can only fulfil the cystine need and, therefore, only a portion of the methionine + cystine requirement. This issue has important practical implications because most feedstuffs included in swine diets are higher in cystine than methionine (NRC, 1998). Thus, diets can be relatively high in total sulphur amino acids but low in methionine. In addition, it has been suggested that a high cystine content may increase the methionine requirement (Kirchgessner et ai, 1994b). Although the question about the maximum proportion of total sulphur amino acids that can be provided by cystine seems simple at first sight, many issues have confounded estimates during the 50 years that this has been investigated. Some of the issues are methodological such as inappropriate response criteria, underestimation of sulphur amino acid content of diets because of improper analysis, and differences in bioavailability of sulphur amino acids in different feedstuffs (Chung and Baker, 1992b). In addition, however, there are two fundamental issues that can confound interpretation of results. First it is important to clarify whether the replacement of methionine with cystine is on a weight or molar basis. Because the molecular weight of methionine (149) is greater than that of cysteine (121), equal weights of these two amino acids provide only 81% as many moles of methionine as cysteine (121/149 = 0.81). Thus, on a weight basis, increasing the methionine : cysteine ratio provides a decreasing number of moles of sulphur amino acids. Almost all estimates of the maximum portion of the methionine + cystine requirement that can be provided by cystine have been expressed on a weight basis and have ignored this issue. The second issue is that the proportion of methionine + cystine that can be provided by cystine is much greater for maintenance than for new tissue accretion. Consequently, the cystine replacement value increases as a pig matures and maintenance becomes a larger proportion of total amino acid need.
An initial estimate of the proportion of methionine + cystine that could be provided by cystine was proposed by Shelton et al.
(1951). Based on their experiment, they concluded that the methionine requirement of 20kg growing pigs was 6.0 g kg-1 in the absence of cystine and 3.0 g kg-1 in the presence of adequate or excess cystine (6.0 g kg 1 added cystine in their experiment). On the basis of these results, they concluded that 'approximately 50% of the methionine can be replaced with cystine in the diet of the weanling pig'. Although this study made an important initial contribution, both the number of animals and the number of treatments was very limited. There was no titration of either the methionine + cystine requirement or various methionine:cystine ratios.
In later work, Curtin et al. (1952a,b) found that the methionine + cystine requirement of 15-kg pigs was approximately 7 g kg-1 of the diet and that 3.8 g kg-1 cystine could replace a corresponding amount of methionine, suggesting that cystine could supply > 50% of the methionine + cystine requirement. Becker et al. (1955) reported that the methionine requirement of 10-kg pigs was 2.5 g kg 1 in the presence of 1.7 g kg 1 cystine. From this, they concluded that cystine could provide 40% of the methionine + cystine requirement. Again, however, in both of these experiments there was no titration of various methionine: cystine ratios and therefore no direct determination of the maximum proportion of cystine that could be utilized.
During the 1960s, two additional papers were published by the research group at the University of Illinois. Mitchell et al. (1968) measured the nitrogen balance of 10-kg pigs. They studied methionine: cystine ratios from 96:4 to 30:70. Although there were no significant differences, there was a tendency for nitrogen balance to increase as cystine replaced methionine. This would be expected because of the increase in the moles of sulphur amino acids added. Based on these results, the authors concluded that 'cystine can replace at least 70% of the methionine need without decreasing nitrogen retention'. Despite this conclusion, there was a 6.5% reduction in nitrogen balance when the proportion of cystine was increased from 57 to 70%. In the following year, the same research group (Baker et al., 1969) reported similar results for the nitrogen balance of 11-kg pigs.
Nitrogen balance tended to increase as the proportion of sulphur amino acids provided by cystine increased from 26 to 66%, although none of the differences were significant. In two growth assays, however, weight gain was reduced when the proportion of cystine was increased from 56 to 66%, leading the authors to conclude that 'regardless of assay procedure, cystine can provide at least 56% of the requirement for total dietary sulphur amino acids'. Differences between the two types of assays were attributed to the fact that feed intake was equalized in the nitrogen balance study, whereas ad libitum access to feed was allowed in the growth experiments.
In a comprehensive series of experiments, German researchers have also investigated methionine-cystine relationships. Based on growth experiments with pigs in weight ranges 30-60 kg and 60-90 kg, Roth and Kirchgessner (1987) concluded that the maximum proportion of sulphur amino acids that could be provided by cystine was 55%. The authors cautioned, however, that at higher performance levels the maximum permissible proportion of cystine may be lower than this. In a subsequent paper, Roth and Kirchgessner (1989) tested methionine: cystine ratios from 36:64 to 64:36. Pig performance improved as the methionine: cystine ratio increased from 36:64 to 40:60 to 45:55. There were no further significant increases in performance as ratios were increased to 50:50, 55:45, 60:40, and 64:36. These data support their earlier research, indicating that cystine can provide up to 55% of the methionine + cystine requirement. However, using quadratic regression analysis, the authors found that peak performance was obtained when the methionine:cystine ratio was 55:45, implying that not more than 45% of the methionine + cystine requirement should be furnished by cystine.
Chung and Baker (1992b) used a purified diet (to avoid issues of different bioavailability of amino acids among different feedstuffs) to study methionine:cystine ratios in 10-kg pigs. In two experiments, methionine:cystine ratios from 100:0 to 40:60 were examined. The authors concluded that no more than 50% of the methionine + cystine requirement could be supplied by cystine.
Thus, estimates of the maximum proportion of methionine + cystine that can be provided by cystine range from 40 to 70%, although the majority of estimates are from 45 to 55%. Some of the difference in estimates may relate to the differences in protein accretion and maintenance needs. The whole-body protein content of pigs contains approximately 1.9 g methionine per 100 g protein and 1.2 g cystine per 100 g protein (see review by Mahan and Shields (1998) of nine different studies). This represents a methionine : cystine ratio of 61:39. Fuller et ai (1989) calculated that the requirements for accretion of 100 g of body protein by 30- to 50-kg pigs were: methionine 1.9 g and cystine 1.7 g, or a methionine:cystine ratio of 53:47. Thus, both of these estimates suggest that less than 50% of the methionine + cystine requirement should be provided by cystine. On the other hand, the optimal proportions for maintenance are quite different. Fuller et al. (1989) estimated that the maintenance requirements in terms of mg kg 1 body weight0-75 per day were 9 for methionine and 40 for cystine. This is a methionine : cystine ratio of 18:82. Part of the reason for the large cystine requirement for maintenance is the high cystine content of hair and other keratin tissues (Baker et ai, 1966). These differences between protein accretion and maintenance are recognized in the NRC (1998) publication, which estimates methionine:cystine ratios of 49:51 for protein accretion and 23:77 for maintenance. Because of these differences, the overall ratio will shift as a growing pig matures and the maintenance needs become an increasing proportion of the overall nutrient needs.
Therefore, based on the results of several experiments and on more theoretical grounds, it seems that in most situations cystine can supply at least 50% of the methionine + cystine requirements. Although in some cases an even greater proportion could probably be provided by cystine, a good practical 'rule of thumb' remains that when formulating diets for growing pigs, one should ensure that no more than 50% of the sulphur amino acid requirement is furnished by cystine.
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