The aim of this study was to determine whether electron transfer from adrenodoxin reductase and adrenodoxin limits the activity of cytochrome P-450scc in mitochondria from the human placenta. Mitochondria were disrupted by sonication to enable exogenous adrenodoxin and adrenodoxin reductase to deliver electrons to cytochrome P-450scc. After sonication, the rate of pregnenolone synthesis was greatly decreased relative to that by intact mitochondria, due to dilution of endogenous adrenodoxin and adrenodoxin reductase into the incubation medium. The addition of saturating concentrations of bovine or human adrenodoxin and bovine adrenodoxin reductase to the disrupted mitochondria gave an initial rate of pregnenolone synthesis that was 6.3-fold higher than that for intact mitochondria. Similar results were observed when 20alpha-hydroxycholesterol was used as substrate rather than endogenous cholesterol. The turnover number of cytochrome P-450scc in sonicated placental mitochondria supplemented with adrenodoxin and adrenodoxin reductase was comparable to that for the purified enzyme assayed under conditions where electron transfer was not limiting. Addition of exogenous adrenodoxin and adrenodoxin reductase to sonicated mitochondria from the pig corpus luteum and rat adrenal had a much smaller effect on pregnenolone synthesis compared with intact mitochondria, than observed for the placenta. We conclude that in the human placenta, electron transfer to cytochrome P-450scc is limiting, permitting pregnenolone synthesis to proceed at only 16% maximum velocity.

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The activities of trypsin (EC 3.4.21.4), chymotrypsin (EC 3.4.21.1), lipase (EC 3.1.1.3) and amylase (EC 3.2.1.1) were measured in different regions of the alimentary tract of ammocoetes from each of the three extant lamprey families. In the southern hemisphere speciesGeotria australis (Geotriidae), and even more particularlyMordacia mordax (Mordaciidae), enzymatic activity was almost entirely confined to prominent diverticular extensions which arise at the oesophageal-intestinal junction. However, in the holarcticLampetra richardsoni (Petromyzontidae), which does not possess a diverticulum, the enzymatic activity was highest in the upper anterior intestine. It is not clear whether the presence of significantly higher amylolytic and lower lipolytic activities in the diverticulum ofG. australis than in the exocrine tissue of the other two species reflects interspecific differences in the composition of their diets. The capacity of exocrine tissue extracts for chymotryptic and tryptic digestion was assayed before and afterin vitro exposure to trypsin and enteropeptidase, their respective catalytic activators. Prior to exposure to these exogenous activators, both proteolytic enzymes were fully active inL. richardsoni, partially active inG. australis and totally inactive inM. mordax. Maximal chymotryptic activity was greater inM. mordax than inL. richardsoni andG. australis. In contrast, maximal tryptic activity was greater inL. richardsoni than inG. australis and was very low inM. mordax. Since trypsin is the only known activator of chymotrypsinogen, the negligible activity of trypsin inM. mordax would appear anomalous unless a trypsin inhibitor is present in the protopancreas of this species. Differences in the distribution of enzymatic activity within the alimentary tract of the three species is discussed in relation to proposed phylogenetic relationships amongst the extant lamprey families.

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