Abstract
It is usually stated that cystine is not present in detectable quantities in normal human blood. 1 The failure of the satisfactory recovery of small amounts of cystine added to blood has been attributed, by Harding and Gary, 2 to difficulties in the preliminary deproteinization necessary prior to cystine determination. They have reported the presence of little, if any, free cystine in cow blood plasma. The amino nitrogen of normal blood (4-7 mg. %) should include small amounts of cystine nitrogen. If loss of cystine in deproteinization could be avoided and if a sufficiently delicate method for the determination of cystine were available, it should be possible to detect cystine in normal blood.
By the use of ultrafiltrates of oxalate-fluoride plasma, mechanical loss of cystine by deproteinization has been avoided and the application of the method of Sullivan to the determination of cystine in such ultrafiltrates has been made possible by the use of the Pulf rich photometer. The details of the preparation of the ultrafiltrates and the determination of cystine will be presented elsewhere in connection with studies of the intermediary sulfur metabolism of experimental animals. Recovery of small amounts of cystine added to blood has been satisfactory.
Application of the above procedure to normal human blood has indicated the presence of approximately 1.0 mg. % of cystine in plasma ultrafiltrates. Samples of blood of one of us (B.H.B.), taken after fasting periods of 12 to 18 hours, have given values of 0.71, 0.80, 0.92 and 0.93 mg. % of cystine in the plasma ultrafiltrate and values of 0.80, 0.91 and 1.05 mg. after meals. The ultrafiltrate of the blood of the same individual, withdrawn 3 hours after the oral administration of 6 gm. of cystine in gelatine capsules, contained 1.72 and 1.40 mg. % of cystine, respectively in 2 experiments. In a third study, in which 7 gm. of cystine were fed, 1.30 and 0.88 mg. % of cystine were present in the plasma ultrafiltrates of blood samples taken 3 and 5 hours later, respectively. In 4 other normal subjects, cystine values ranging from 0.82 to 1.13 mg. were obtained.
The plasma ultrafiltrate of the blood of a cystinuric patient (S.P.) 3 contained 1.13 mg. % of cystine, a value slightly higher than our average normal value, but, we believe, within the upper limit of the range of values for normal human blood. To this subject, 5.6 gm. of methionine were administered in 2 equal portions at the morning and midday meals. The plasma ultrafiltrate of a sample of blood withdrawn at 3 :45 P. M. contained 0.92 mg. % of cystine. It may be noted that, while the isolation of cystine from cystinuric blood has been claimed, 4 , 5 no quantitative data have been reported.
It should be noted that no attempt was made to determine the possible presence of cysteine in these ultrafiltrates. The values reported represent “total” cystine, i. e., values obtained by the application of the Sullivan procedure to the ultrafiltrates after reduction with cyanide.
This research was made possible by a grant to one of us (H.B.L.) from the Faculty Research Fund of the University of Michigan for the study of cystinuria.
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