The dissociation of avidin–biotin complexes by guanidinium chloride

Avidin molecules in which a fraction of the four binding sites were occupied by biotin did not dissociate completely in 6.4m-guanidinium chloride. Only unoccupied subunits dissociated. The remainder recombined to form the tetrameric avidin-biotin complex. The rate at which unoccupied subunits were unfolded and dissociated was only decreased by one-half in species in which three of the four binding sites were occupied by biotin. These results can be explained by assuming that unfolding of unoccupied subunits followed by dissociation from the tetramer is initiated by penetration of guanidinium ions into the binding site and disorganization of this region of the subunit. When a site is occupied by biotin this pathway is blocked and the subunit does not unfold. Each subunit behaves independently and is not markedly stabilized when neighbouring subunits are occupied.     

Breast-feeding and respiratory syncytial virus infection.

The pattern of breast-feeding in 127 infants admitted to hospital with respiratory syncytial virus infection was compared with that in 503 age-matched controls. Thirty per cent of children with infection had been breast-fed compared with 49% of controls. The approximate relative risk of being admitted to hospital with respiratory syncytial virus infection if not breast-fed was 2.2. Several other factors were also considered, including an assessment of maternal care and home environment; the mother''s age, marital state, and smoking habits; the number of siblings; and gestation. Adverse factors were all associated with an increased risk of admission with infection, but breast-feeding still appeared to provide protection after controlling for these other factors in turn. These findings provide further support for encouraging mothers to breast-feed their infants and should prompt further studies into the immune status of mothers and into the nature of the protective factors in their breast milk.