THE FUNCTION OF CYSTINE IN WOOLPRODUCTION.

New Zealand Journal of Agriculture, Volume 43, Issue 4, 20 October 1931, Page 262

THE FUNCTION OF CYSTINE IN WOOLPRODUCTION.

O. H. Keys,

M.Sc., Farmers’ Union Research. Scholar, Massey Agricultural College, Palmerston North.

The necessity for research into wool problems was never more urgent than at the present time. If wool is to retain its prestige as a textile material a complete understanding of its chemical and physical attributes is most desirable from the point of view of both the producer and the manufacturer. In the following review an attempt is made to indicate in broad outline, for the benefit of wool-growers, the basis of the research work now in progress at the Massey Agricultural College on the biochemistry of the fleece and its production. When this work is correlated with other research being carried out at this college and in other parts of the world, . much information of economic and practical importance should result. Up to the present time very little attention has been paid to the nutrition of the sheep in relation to the quality of wool. Methods of improving quality have hitherto been based almost entirely upon selective breeding. While this must ever be a first consideration with the farmer, yet the question of feeding is of the utmost importance/

This article deals with a particular aspect of animal nutritionthe role played in wool-production by a substance which has in recent years been brought into the forefront of biochemical investigation.

It has been known for a long time that wool contains a large percentage of sulphur. Recently, however, the fact has emerged that there is a relationship between quality and sulphur content, it having been found from the analyses of a large number of wools that there is less sulphur in hairy medullated wool than in samples of better quality. Work done by the writer in this country shows that this applies to the New Zealand Romney breed ; this fact has now been confirmed by Sidey 1 . Sulphur content, therefore, becomes of great importance in the study of the fibre. Sulphur confers upon the wool elasticity, strength, and resistance to decomposition) and this circumstance ■ has been likened to the vulcanization of rubber by the incorporation of sulphur. ■ ‘

Clean scoured wool is composed entirely of a protein, keratin. Keratin is also the chief constituent of hair, horns, and hoofs, and is singular among the whole class of proteins in its durability. Of the twenty amino acids which compose all proteins, one of . them, cystine, occurs in a remarkably high proportion in keratin. Very nearly the whole of the sulphur in wool is present combined in the form of cystine, which occurs there in greater amount than any other amino acid and appears to be the most characteristic constituent. Since the core of medulla, which is present in all hair fibres and represents the main fault in these fibres, appears to contain a lower percentage of sulphur (and hence of cystine) than pure wool keratin, a theory of keratinization has been advanced. It is suggested that if the growing fibre has not received the necessary supply of cystine the cells in the centre fail to become converted into keratin,. and are therefore protruded upwards,

and appear as medulla. One may therefore speak of the “ cystine stimulus” at the root of’the growing fibre; when this is inadequate the wool will suffer either in quality or quantity or in both.

It becomes obvious that the sheep requires a considerable amount of cystine for the growth of wool, in precisely the same way that a contractor must have cement for the production of concrete. In addition, it is possible, as has been pointed out, that the quality of the fibre is dependent upon the cystine supply.

Passing reference may here be made to the fact that cystine possesses marked absorptive power for ultra-violet light ; its occurrence in large quantities in feathers, animal hairs, wool, and epidermal scales indicates that the evolutionary process has retained this substance as a protection against the toxic rays of the sun.

There is abundant evidence that no animal can manufacture cystine from simpler substances for its own requirements. It must be obtained in a preformed state in the diet. Now, dry wool actually contains, in round figures, 13 per cent, of cystine. That the importance of this study cannot be ' overestimated is evident when it is realized that the usual pasture plants contain only very small amounts. Indeed Aitken 2 , working at Otago University, was unable to demonstrate its presence in grass at all. Yet since every known protein contains cystine, even though it may be present only in minute amounts, and since the sheep is dependent upon the pasture for its cystine supply, it seems probable that the demonstration of its presence in grass merely awaits the development of a suitable analytical technique. Emphasis should be laid on the fact that inorganic sulphur as supplied in' licks, &c., cannot take the place of cystine, nor can it be incorporated by the sheep into its growing wool; its role in nutrition is of minor importance and need not be further mentioned in what follows. We are here concerned with the. utilization of cystine-sulphur as supplied by the proteins in the diet'of the animal.

Causes of Cystine Deficiency in the Wool Fibre.

The failure on the part of a sheep to incorporate sufficient cystine into the growing fibre may be due to a variety of factors, hereditary and environmental. It is desirable briefly to consider these factors separately, although their several effects are probably interrelated in a complex manner. Other agencies of which at present we have no knowledge may also interfere.

(1) Hereditary Factors.— Different breeds and different individuals presumably possess in varying degrees the power to supply the optimal quantity of cystine to the fibre as it grows from the follicle. In the Merino, which has been bred for wool for hundreds of years, medulla is conspicuously absent or rare ; the Romney, on the other hand, frequently grows a fleece. in which coarse hairy fibres abound. Again, .within some breeds the tendency to produce medulla varies from one animal to another, due to hereditary differences.

(2) Seasonal Factors. — At certain times, when wool is being grown at a fast rate, the “ cystine stimulus ” may be unable to keep pace with the rapidly growing wool. This statement rests

upon at least two observed facts : (a) When a given fleece contains both medullated and non-medullated fibres, the tendency towards medullation is greatest in those which have grown the fastest ; (6) immediately subsequent to shearing, the sheep is obliged to produce wool rapidly for protection ; there is a marked tendency towards medulla-formation at this period of rapid growth. This fact is illustrated by the occurrence of medulla at the tips. Moreover, bands of medullation ' are frequently found in definite horizons below the tip, suggesting a seasonal effect.

(3) Quantitative Factors : Inadequate Supply of Cystine. This vital factor has been mentioned above. Vegetable proteins are usually poor in cystine content, and, since wool itself contains 13 per cent, of cystine, any failure in the supply of the necessary quantity must affect the fleece adversely. It is not improbable that the proteins of the rapidly growing succulent spring grasses are lower in cystine content than are those of the grasses at other seasons. This would readily explain a tendency towards medulla-formation during the spring period. The capacity of a pasture to produce cystine is probably a determining factor in wool-production.

(4) Physiological Factors.— lt is reasonable to anticipate that the general health of a sheep will affect its ability to produce the desirable type of fibre. In particular, attempts to correlate growth of wool with the activity of the glands of internal secretion have been singularly successful. More attention has been paid to the activity of the thyroid gland in this connection. Not only do abnormal conditions of the thyroid result in profound changes in the well-being of an animal, but it is also believed that this gland has a specific effect in controlling the growth of wool. This is of especial importance in view of the presence of several goitrous districts in New Zealand. The availability of iodine may therefore be another limiting factor in wool-production. Removal of the thyroid from sheep has caused the growth of a coarse wool which was easily pulled out, and the weight of such a fleece was only two-thirds that of normal control sheep 3 . In another experiment, extending over three years, feeding extra iodine gave an increase in the number of lambs, the live weight of the animals, and the yield of wool. The explanation of these findings is, in the opinion of the writer, that the failure of the thyroid gland to function efficiently probably prevents the animal from utilizing completely the cystine in its diet for wool-production. It must be emphasized, however, that the addition of iodine in the form of iodized licks, &c., is likely to be beneficial only in districts where there is a tendency towards goitrous conditions ; moreover, an excess of iodine may produce serious results. The availability of iodine is therefore probably secondary in importance to the availability of cystine.

Physiological Chemistry of-Cystine.

The physiological chemistry of cystine may be discussed in two sections, the first comprising the general requirements of the animal body and the second the specific demands for particular purposes. This distinction is by no means arbitrary, and' the two divisions, though closely interrelated, will be considered separately.

(1) General Requirements. Modern biochemical research indicates in a very definite manner that the sulphur atom is responsible for the fundamental chemical reactions taking place in living protoplasm, acting as it does in every single cell of animal and plant tissue as ' a donor and acceptor of oxygen. Here the sulphur occurs combined as glutathione, which is itself a compound of cystine and glutamic acid. Animals obtain their supplies of amino acids only by the breaking-down of the proteins of their diet. Certain of the amino acids are absolutely indispensable for maintenance, and growth ; cystine is one of these. Thus, if there is an inadequate supply of cystine in a diet which is otherwise complete, the animal will progressively decline and eventually die, having in the meantime actually utilized some of the proteins of its own tissues to supply the deficiency. The regular ingestion of sufficient cystine is therefore an absolute necessity for the performance of normal physiological functions. As to the actual form in which cystine is absorbed, no definite evidence as yet exists. At this juncture it is worth while to indicate the entrance .of another factor which complicates the problem : cystine itself exists in several different forms, but it appears that only one of them (Zaevo-cystine) is capable of being assimilated by animals. The failure to. exclude the other forms from cystine preparations has in some cases vitiated, or at least rendered doubtful, the conclusions reached by other workers from its inclusion in artificial diets.

(2) Specific Requirements.— Superimposed upon the relatively small quantities indispensable to the normal functioning of the body processes, there are specific requirements of cystine for the elaboration of such keratin structures as horns, hoofs, hair, and wool. In the absence of a plenteous supply of cystine for both general and specific requirements, there arises between the two sets of functions a state of competition, in which there are two possibilities : either the demands of the vital tissues are fulfilled to the detriment of the wool follicles, or the demands for large quantities for wool growth take precedence over the requirements of the body. Here there is a diversity of opinion, but, although the available evidence seems to favour the latter possibility, no definite statement can be made. Several investigators have reported loss of hair in rats fed on cystine-deficient diets, and the addition of cystine to a previously deficient diet has repeatedly produced a heavier and denser coat. In one such case 4 the hair was analysed for its cystine content, which was found actually to have increased. The theory of keratinization outlined above (which, it should be noted, was advanced from separate evidence) is therefore strongly supported by feeding trials. Furthermore, ,it has recently been shown by a French investigator that prior to keratinization there is an accumulation of sulphur in the tissues of the horse. The writer of the present paper, in examining the roots of plucked wool fibres, has demonstrated the presence of significant quantities of a substance very closely related to cystine. We are therefore at present satisfied with the theory of keratinization and are using it as the basis of the inquiry into the biochemistry of wool-production.

Aim of the Research Work at Massey Agricultural College. In no previously recorded case have any of these conclusions been applied to the elimination of medulla in wool, but this new field is now

being explored at this College. In Australia, it is true, supplementary feeding of sheep under drought conditions with blood-meal (containing about 3 per cent, of cystine) has resulted in a slightly heavier fleece 5 . While this result is decidedly encouraging, the specific action of cystine in increasing the fleece weight was not proved owing to -the presence in the supplementary feed of important food constituents other than cystine. The work now in progress aims at tracing the effect of pure laevocystine upon the incidence of medulla, fleece weight, growth-rate, tensile strength, and fibre diameter. In. the event of definite results being obtained, investigations will be undertaken into the most economical methods of providing regular and adequate supplies of cystine.

REFERENCES. (1) Sidey, Jour. Textile Institute, 1931, Vol. 22, p. 370. (2) Aitken, Biochem. Jour. .1930, Vol. 24, p. 250. • (3) Simpson, Quart. Jour. Exper. Physiol. 1924, Vol. 14, p. 185. (4) Lightbody and Lewis, Jour. Biological Chem. 1929, Vol. 82, p. 663. (5) Pastoral Review, 1930, Vol. 40, p. 983 (October).