Mitochondrial matrix granules in soft tissues: II. Isolation and initial characterization of a calcium-precipitable, soluble lipoprotein subfraction from brown fat and liver mitochondria

Abstract

Isolated mitochondria from lamb brown fat and rat liver were subject to hypotonic shock to free the matrix granules. Total mitochondrial membranes were collected by a two-stage high-speed centrifugation. By addition of calcium ions to the supernatant, membrane-like structures were formed and collected by further centrifugation. This calcium-precipitable fraction was a phospholipoprotein with very low or absent marker enzyme activities for inner and outer membranes but with the same relative amount of cardiolipin as the inner membrane. It contains fewer proteins in somewhat different proportions than the total membrane fraction. A single incorporation experiment indicated that the calcium-precipitable fraction might be enriched in products of mitochondrial protein synthesis. The available evidence suggests that matrix granule material forms the major part of the calcium-precipitable fraction and that, as previously hypothesized, matrix granules are involved in the assembly of the mitochondrial inner membrane.