No abstract

1. Studies on samples of different tissues collected during three seasons from Juglana regia trees growing in several districts round London showed, in the endocarp, remarkably high concentrations of vitanin C, which may be used in lignification. This vitamin C probably originates mainly in photosynthetic centres in the leaves.2. Examination of the intervening tissues showed that physiological conditions (acidity, concentration of enzymes and of vitamin C) militate against translocation of vitamin C as such from leaf to nut. The experimental findings favour an alternative hypothesis, which will be considered later.

No abstract

The reaction is strictly reversible and will go to completion only under conditions where the concentration of RS-is small, as for instance at pH 7 (Stricks & Kolthoff, 1951; Cecil & McPhee, 1955). Insulin has two disulphide bonds linking the A and B chains and a third intra-chain bond linking residues 6 and 11 (referred to below as the 6-11 intra-chain bond) on the A chain (Ryle, Sanger, Smith & Kitai, 1955). Cecil & Loening (1960) studied the reaction of these disulphide bonds with sulphite. They found that 'the two inter-chain bonds react with sulphite at pH 7 causing the insulin to split into its component A and B chains. The 6-11 intra-chain bond does not react with sulphite alone but it will react with sulphite in the presence of phenylmercuric hydroxide. Partial reaction occurs in the presence of urea or guanidine. The explanation suggested by Cecil & Loening for this difference in the reactivity of the inter-and intra-chain disulphide bonds was that the structure of the 6-11 intra-chain ring is stabilized by hydrogen bonds (or other non-covalent bonds) rather than by the disulphide bond. This would have the effect of shifting the equilibrium of the reaction with sulphite to the left since the ring would have no tendency to open when the disulphide bond is split. Phenylmercuric hydroxide would block the reverse reaction by combining with the thiol formed. Urea and guanidine would reduce the stability of the 6-11 intra-chain ring by breaking the non-covalent bonds. In these cases the disulphide bond would be expected to react. Bovine serum albumin has 17 disulphide bonds and these appear to behave similarly to the 6-11 intra-chain bond in insulin. Kolthoff, Anastasi & Tan (1958, 1959) found that there was no appreciable reaction with sulphite alone but that all 17 bonds reacted with sulphite in the presence of mercuric chloride. In the presence of guanidine 12-13 bonds reacted with sulphite. Hunter & McDuffie (1959) have shown that the molecular

No abstract