THE X-ray diffraction photographs usually obtained from apparently nonfibrous proteins have so much in common with one another and with the photographs given by certain natural protein fibres when disoriented that the inference seems clear that all proteins at some stage of their existence are fibrous in the molecular sense [Astbury, 1933; 1934,1,2]. Recently [Astbury and Lomax, 1934, 1, 2;1935] this concept has been expressed as a generalised interpretation of denaturation in the conclusion that the two more stable and insoluble states of protein structure, the fibrous and the denatured, are based on fundamentally similar modes of molecular arrangement; that, in fact, the denatured state2 is essentially a fibrous state inasmuch as it always consists of peptide chains, often fully extended, and aggregated after coagulation in parallel bundles, as in fibroin [Meyer and Mark, 1928], f3-keratin [Astbury and Street, 1931; Astbury and Woods, 1933], /3-myosin [Astbury and Dickinson, 1935], fibrin [Katz and De Rooy, 1933] etc. Itwas found that heat-denaturation of the albumins, for instance, merely makes the X-ray photograph more like that of a random arrangement of fibres of/-keratin (stretched hair, horn, etc.; cf. Plate V, Figs. 1 and 3). ,B-Keratin is built from almost fully-extended polypeptide chains linked side-to-side, firstly by combinations between their side-chains ("side-chain linkage "), and secondly in a direction at right angles [Astbury and Sisson, 1935], through attractions between the =CO and =NH groups of neighbouring main-chains ("backbone linkage"). The three-dimensional structure is that of a pile of polypeptide "grids ", the average distance apart of the main-chains in the plane of each grid being about 9*8 A. ("side-chain spacing") and the distance between the grids being 4*65 A. (" backbone spacing "). These two spacings correspond respectively to the two reflections, 001 and 200, seen on the equator of Fig. 4: they are the two prin'cipal side-spacings of the ,B-keratin crystallites, whilst the reflection 020 (spacing about 3*4 A.) gives the average length of an amino-acid residue in the direction of the main-chains. Fig. 4 is a "fibre photograph" of crystallites all pointing roughly in the same direction (the fibre-axis) and taken with the X-ray beam perpendicular to this direction; but when the crystallites are 1 Beit Scientific Research Fellow. 2 The seed globulins, at least, appear to pass through an intermediate fibrous state before reaching complete denaturation. It is possibly better therefore to reserve the word "denatured"

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The major protein component of muscular tissue, myosin, has occupied the attention of many investigators because of its extreme importance as the contractile element of living. muscle. When isolated from the tissue it has many of the properties of the globulins, but its extreme molecular asymmetry gives rise to interesting properties which on the whole are anomalous to the corpuscular proteins. Past research has been devoted mainly to physico-chemical studies, and amongst these the investigation of double refraction of flow [Muralt & Edsall, 1930; Edsall & Mehl, 1940] and the interpretation of structure by X-ray methods [e.g. Boehm, 1931; 1933; Astbury & Dickinson, 1940] are perhaps most important. Some work has also been carried out upon its solubillty [Bate Smith, 1934], its chemical composition [Sharp, 1939; Bailey, 1937], upon the comparative biochemistry If future research confirms this view, it must be conceded that the original work of Liubimova & Engelhardt is a most fundamental contribution to muscle biochemistry. (121) K. BAILEY EXPERIMENTAL Preparation and properties of myosin Preparation of various myosins. The tissues used were the hind-leg and longissimus dorsi muscles of the rabbit, the pectoral muscle of chicken, the leg muscle,of the horse, the hind-leg muscle of the toad and frog, and the trimmed heart muscle of the pig and rabbit. The rabbits and chickens were killed by stunning and bled via the inferior vena cava; frogs and toads were pithed after leaving the animals in the cold room at 00. The muscle of those animals killed in the laboratory could be excised and minced within 10 min. after death, but material from the slaughterhouse was usually obtained about an hour after killing. In one instance, however, a sample of horse muscle was acquired within 15 min. after bleeding. Immediately after mincing, the tissue was added to 5 vol. of ice-cold salt solution containing KCI (0.5 M) and NaHCO3 (0.03 N). This extractant has been adopted by Greenstein & Edsall [1940] in an improved method for the preparation of myosin. Solid NaHCO3 was now added at intervals to maintain the pH at 7-7 5, and after stirring slowly for an hour at 00, the stroma was centrifuged down. The opalescent extract was filtered clear through a 2 in. layer of paper pulp (saturated with extractant) and diluted with 20 vol. of ice-cold water. Dilute HCI was now added to adjust the pH of the medium to 6-8-7-0, and after settling, the supernatant liquor was decanted and the myosin centrifuged down. For purification it was redissolved by the addition of solid KCI to a molarity of 0 5 and sufficient NaHCO3 to maintain the pH at 7. Without such addition, the myosin tends to become increasingly acid as purification proceeds, and although it remains soluble in salt, its enzymic activity diminishes, not by loss of an enzyme component, but by inactivation. After 3-4 precipitations and a final filtration through pulp, the protein may be stored in 0 5 M KCI at pH 7 and 00, in presence of a trace of toluene. N and P contents of rabbit myosin...

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