1. If two compounds are substrates for a single enzyme, and do not form any ternary complex with the enzyme or combine directly with each other, then the total initial rate of reaction for a mixture of the two compounds may be greater than the rate for either compound alone, or may lie between the rates for the compounds alone. It is the concentration of the compound with the higher maximum velocity that determines which applies, and there is one concentration of the compound of higher maximum velocity at which the total rate of reaction is independent of the presence or absence of the substrate of lower maximum velocity. The values concerned are derived. 2. An example is given of 5alpha-androstan-3-one and 5alpha-androstane-3,16-dione as substrates competing for a hydroxy steroid-NAD oxidoreductase (EC 1.1.1.53).

Summary.-Electron spin resonance spectra have been obtained from samples of frozen whole blood or separated blood cells and plasma. Blood samples were obtained from human controls having no diagnosed malignancy and from patients with a variety of benign and malignant tumours.The characteristic spectrum from control blood shows two main lines with g values of 4-2 and 2-049. Several smaller lines can also be observed. The line at g = 2-049 may be due to the copper protein ceruloplasmin. Although no qualitative differences could be found between the spectra from controls and cancer patients, samples from patients with certain types of tumour showed a significant increase in size of the g = 2-049 signal above control values. This was most noticeably the case with Hodgkin's disease and to a lesser extent with cancers of the breast. Squamous cell carcinomata, taken as a group, did not show an elevation in average size of the g = 2-049 signal. In this latter group, however, there were some indications that the effects of chemotherapeutic treatment could be followed during the early stages of such treatment. Examples are given in which onset of treatment with various cytotoxic agents was associated with reduction in size of the g = 2-049 signal.

Summary.-Electron spin resonance studies have been made of caeruloplasmin and iron transferrin levels in whole blood of healthy controls and patients with a variety of malignant conditions receiving various forms of treatment.A small difference was found in caeruloplasmin level between normal males and females, although normal females receiving contraceptive steroids had an elevated level. No difference was found in iron transferrin level. Various conditions increased the caeruloplasmin and some also decreased the iron transferrin level in patients with malignant disease. These included surgery and the approach of a terminal phase of disease. Once allowance for these factors was made, the remaining small differences in Cu and Fe between patients with either squamous cell carcinoma or breast cancer and controls appeared to have no clinical significance.

Crystalline 20p-hydroxysteroid : NAD oxidoreductase preparations from Streptomyces hydrogenans were shown to have 3a-hydroxysteroid : NAD oxidoreductase activity. Compounds of both the 5a-androstane and 5a-pregnane series served as substrates.Among the compounds tested, no evidence was found of 3P-hydroxysteroid : NAD oxidoreductase activity. The 3a-hydroxysteroid : NAD oxidoreductase activity did not extend to compounds having 5P-H, a 4,5 double bond, a 5,lO double bond, or 1,2 and 4,5 double bonds together. I n the 5a-H series, a 1,2 double bond prevented the compound from serving as a substrate.Substituents a t C-11, -16 and -17 altered the kinetic constants for the 3a-hydroxysteroid : NAD oxidoreductase reaction, and in the case of an IIP-hydroxy or 11-0x0 group the effects were similar to the effects reported for these groups for 20p-hydroxysteroid : NAD oxidoreductase activity.Kinetic data were consistent with the view that two types of 3a-hydroxysteroid : NAD oxidoreductase were present which involved different active centres, neither of which was the active centre for the 208-hydroxysteroid : NAD oxidoreductase activity. It is not yet established whether these enzymic activities are associated with the same or different molecules. The 3cr-hydroxysteroid : NAD oxidoreductase activities do not correspond to those of previously characterised enzymes (3a-hydroxysteroid dehydrogenase and a-hydroxycholanate dehydrogenase. /3-pregnan-3,20-dione [7] and use has been made of this activity for analytical purposes [S-lo]. A limited type of 3a-hydroxysteroid : NAD oxidoreductase activity in these crystalline preparations was discovered in 1965 [lOa], and indicated the importance of characterising the preparations further. Studies on aspects of the specificity are now reported.

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