The effect of supplementary cystine and energy on the utilization and distribution of cystine in merino sheep

Abstract

Twelve mature Merino wethers were offered a diet which supplied 0.44 MJ of digestible energy per kg^0.75 liveweight daily, and sufficient nitrogen and sulfur to maintain positive balances of these elements. The wethers were given supplements within a 22 factorial design of L-cystine (2 g day^-1) infused into the abomasum and/or volatile fatty acids (VFA) infused into the rumen to increase the availability of digestible energy by 15%.

Although intra-abomasal infusions of cystine increased the retention of sulfur by the wethers (P < 0.05), neither this nor the VFA treatment had any effect on the retention of nitrogen or the proportions of digested nitrogen and sulfur retained by the sheep.

The non-ammonia nitrogen and sulfur flowing through the abomasum of the wethers consuming the basal ration represented 105 and 112% of the dietary nitrogen and sulfur respectively. The treatments had no significant effect on the flow of these elements, nor on the digestion of organic matter in the reticulorumen. The intra-abomasal infusions of cystme increased the entry rate of cystine into the plasma from 0.55 to 1.12 mg min^-1 (P < 0.05). The entry rate was linearly related to the concentration of free cystine in plasma, both within and between treatments (P < 0.05). Consequently, the sheep receiving the supplementary cystine had greater concentrations of free cystine in their plasma (7.1 v. 16.3 .µM; P < 0.05).

After L-[³⁵S]cystine was infused intravenously for 14 h, c. 80% of the infused ³⁵S was contained in the following tissues: skin (31%), carcass muscle (12%), small intestine (14%), plasma (12%), liver (8%) and kidneys (3%). The supplementary cystine increased the proportions of the infused ³⁵S taken up by muscle and liver, had no effect on that taken up by the skin, and reduced the proportion taken up by kidneys, intestine and plasma (P < 0.05). The levels of ³⁵S per unit dry weight of tissue were greatest in the kidney and least in muscle (biceps femoris); the level in the kidney being 34 times as great as in muscle.

Tissues varied in both the proportions of ³⁵S in various chemical forms and the levels of ³⁵S in these forms. The supplementary cystine generally influenced both traits, but the provision of additional energy had no significant effects. In addition, the responses of both the proportions and levels to the supplementary cystine frequently varied among the tissues, leading to significant interactions. When cystine entering the plasma was taken into the tissues, it was predominantly contained in the proteins of the tissues. Neutral compounds, including cystine, in the acid-soluble fraction from tissues represented the second major form. Acidic and basic compounds in the acid-soluble fraction were minor products. The acidic, however, were quantitatively more important, and more responsive to the treatments, particularly in the liver and kidney. ³⁵S bound by disulfide linkage to the tissue proteins and ³⁵S as inorganic sulfate represented only small proportions of the total ³⁵S in the tissues.

The concentration of cystine in the acid-soluble fractions of the tissues varied among the tissues studied, being greatest in the liver (138 µM). Cystine was not detected in the protein-free filtrate from muscle. The supplementary cystine increased the concentration of cystine in the protein-free filtrate of skin (7.2 v. 22.8 µM; P < 0.05).

While the supplementary cystine influenced most of the measurements of the availability and utilization of cystine by the various tissues, additional energy, supplied by the VFA infusion to the rumen, had little effect on these measurements. The important role of the skin and the minor role of the large mass of carcass muscle in the utilization of cystine are indicated by the results.