Background-The etiology of respiratory distress syndrome (RDS) is multifactorial and/or multigenic. Surfactant protein A (SP-A) and/or SP-B genetic variants have been identified as risk or protection factors for RDS.

Rat liver mitochondrial-lysosomal fractions were separated on gradients of colloidal silica. Lysosomal enzymes were distributed bimodally. The dense peak (1.117 g/ml) was nearly free from contaminants; beta-N-acetyl-D-glucosaminidase was enriched nearly 60-fold. By contrast, the buoyant peak (1.085 g/ml) co-sedimented with mitochondria, microsomes, peroxisomes, and Golgi particles. Decreasing the amount of protein layered on the gradient medium or dispersing a full sample through it shifted lysosomal marker from the buoyant to the dense peak. Thus the majority of lysosomes in the two peaks appeared to have equivalent densities. Electron microscopic examination of particles separated from gradients with layered samples showed that the dense peak contained most of the dense bodies, whereas the buoyant peak was relatively enriched in autophagic vacuoles. Dispersion, however, shifted autophagic vacuoles from the buoyant to the dense peak without affecting the distribution of dense bodies. We conclude that the bulk of buoyant particles act as a sieve to retard the density equilibration of autophagic vacuoles without specifically affecting other lysosomal enzyme-containing components.

No abstract

Lysosome-enriched samples from normal fed rat livers have been shown to separate into dense and buoyant bands when layered on self-generating gradients of colloidal silica. The hypothesis that the buoyant peak is comprised largely of autophagic vacuoles that are selectively restrained by sedimenting mitochondria was tested in livers subjected to conditions known to accelerate autophagy or suppress it. In all cases, a sharp peak of beta-N-acetyl-D-glucosaminidase activity was evident when autophagy was present but was undetectable when it was eliminated or virtually eliminated. Moreover, the sensitivity to osmotic shock of buoyant peak particles, but not those of the dense band, was increased after autophagic stimulation. The total amount of enzyme that moved to buoyant fractions with the induction of autophagy generally correlated with the degree of autophagic acceleration. However, with maximal autophagy, the buoyant peak was broad, and only 70% was capable of being dislodged and returned to the dense peak by dispersing samples in the gradient medium and/or reducing the sample size. On the other hand, at roughly half-maximal stimulation and lower, the buoyant distribution was sharply defined and completely reversible. These findings support the above hypothesis and suggest that the buoyant peak could be used as a measure of autophagy in the physiological range of regulation.