Studies on the particulate components of rat mammary gland. 3. The relationship between enzyme activity and particle counts in mammary gland suspensions

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Abstract
Succinic oxidase activity in suspensions of rat mammary tissue increases sharply at parturition and reaches a maximum value midway through lactation. The identical patterns of change of activity in the whole-tissue suspension and in the large-particle fraction, together with the high recoveries of activity in the large-particle fraction, indicate that the succinic oxidase activity of rat mammary tissue is very largely localized in this fraction. The increase in succinic oxidase activity at parturition and in early lactation is probably the result of three effects: an increase in mitochondrial numbers; an increase in mitochondrial size; the removal of inhibitory factors at parturition. The large-particle count procedure indicated a rapid decrease in the number of particles present over parturition. It is suggested that this was the result of a rapid decrease in the number of particles other than mitochondria occurring simultaneously with a smaller increase in mitochondrial numbers. Interrelationships between RNA, DNA, tissue N, succinic oxidase and the large-particle counts are discussed.