Changes in protein carboxyl-methylase isoenzymes during testicular development in the rat

Measurements of total rat testicular protein carboxyl-methylase (PCM) activity at various ages ranging from 10 to 90 days showed a rapid (fivefold) increase between 20 and 30 days, coincident with the occurrence of haploid germ cells. When partially purified testicular enzyme, at different ages, was fractionated by isoelectric focusing, the activity of the individual PCM isomers displayed clear differences in the time at which maximal activity was reached. The most acid PCM isoenzyme (pI 6.1) was the dominant isoform in the immature testis and exhibited the highest increase in activity between 20 and 30 days of age. During the same age interval, the activity of the other two isoenzymes (pI 6.7 and 7.4) increased only slightly. At 50 days, while the isofocusing profile revealed no additional increase in the activity of the acid isoform (pI 6.1) and only a doubling in the activity of the second isomer (pI 6.7), the most basic isoenzyme displayed a drastic increment in specific activity (approximately sixfold when compared to 30 days). This indicates that the PCM isozymes have different cellular localization in the seminiferous tubules. The acid isomer (pI 6.1) appears to be associated to the early stages of spermatogenesis and may represent both germ cell and somatic cell PCM. The basic isomer (pI 7.4) may be specific for germ cells (spermatids), and is most likely associated with the later stages of germ cell maturation.