## Abstract

The combination of the body wall and perienteric fluid haemoglobins of the pig round worm, Ascaris lumbricoides, with oxygen, carbon monoxide, and nitric oxide has been followed over a range of temperature and pH values. The rates for the perienteric fluid haemoglobin were NO: 4$\cdot $7 $\times $ 10$^{6}$ M$^{-1}$ S$^{-1}$; O$_{2}$: 1$\cdot $5 $\times $ 10$^{6}$; CO: 1$\cdot $7 $\times $ 10$^{5}$. The rates for body-wall haemoglobin were 30% less in each case. The rate of dissociation of carbon monoxide is 0$\cdot $018 S$^{-1}$ at 20 degrees C for the perienteric fluid haemoglobin and 0$\cdot $039for the body-wall haemoglobin. These rates are similar to those for mammalian haemoglobin. As was already known, the velocity of dissociation of oxygen from these haemoglobins is much lower. The velocity of dissociation for perienteric fluid haemoglobin was difficult to study because of the presence of methaemoglobin. The rates of all the reactions studied were close to those which would be predicted from the activation energies if the reactions were assumed to be kinetically normal. Over-all equilibrium constants for the reactions with oxygen and carbon monoxide have been calculated from the kinetic data. The half-saturation value for the perienteric fluid haemoglobin at 20 degrees C is 0$\cdot $0015 mmHg O$_{2}$. The value for the body-wall haemoglobin is 0$\cdot $11 mm. The ratio of the oxygen and the carbon monoxide equilibrium constants, M, is 0$\cdot $075 for perienteric fluid haemoglobin and 0.82 for body-wall haemoglobin. Neither of these values conforms to the relation between log$_{10}$ M and the span of haemoglobin observed for mammalian haemoglobins and myoglobin.