R n m t i c ribonucleaae (RNaec) is a w e b d i e d tnzyme with respect to ita mobcukr structw and activity parame tern However, until n a n t l y , no a-mpt has been made to aama its metabolic a- n i k a in diffemnt animrL nor to dispel implicatianr that the RNISC coocentratim in the ptmcmaa of the ODW is represent.- tive of that in d vertebmtes. E. N. Zmdzian and E. A. Barnard (Arch.

Bioe)um. Biophus. 122,699 (IS7)) meme ured the arrcartrations d EtVlse, trypein, and chymatrypsia in paacreat.ic t h e of several vertebrate epecies. They found that trypin and chymmpsin were uni- formly abundant in all species studied, while RNsst concentmticm varied m a r k - abty and waa virhrally absart in many cams. Tbt fact that R N e weu particularly high in ruminante and raa f a d m high or moderate levels in all of the ungulatc spsciee mggestd that berbivoras diet or the presence of large. gatdamintestinal microbial populations -were functiaMuy asmciatcd with the enzyme abundance.

Barnard (Nature 22I.310 ( 1 S 9 ) ) has re- cently measured RNase activity m pan- m t i c t h e of eweral additianal speciat. Of the 50 vertebrate epccies w w studied, only ungulates. rodent.. and herbivoram .marsupials poeeea, hi& lev& of pan- atic mast. All d e r species investigated have low or moderate levels of the enzyme. On the bmnia of these findings and a few mttabolic -dies. Barnard propoees a modificatioo of the mminant nitrogen cy- de to include and emphasize the role of the nucleme in the breakdown of mi- bial R N A and the cycling of phoephorus.

In all cases, enzyme concentration is u- pteesed as the equivalent of bovine RNase

per gram of wet weight of -. It sbauld here be pointed out that no a t tcmpt wan made to msssure the &-tic samtory m+ or the tamover time d ri-

meterr would bccpl to be important am- sideratiore if tbe relative amounts of RNue in are to be credited with metabolic signdhnce in the gut hamen.

In an attempt to correlate pnueat ic

d to utilize aogcaour nudeic acid.

three rabbits. tao monLcyq three mice. and a cat. The levels of RNaae in these four species were: O.& 2, 395, and 0.5 M. pcr gam d pancnas. rapccrivaY. In most cases, tbe animals were fedunlabeled RNA for w v c d clam before the tracer d<re wae givcn to avoid tbe amfounding dect of W b l e variable ram of induction. Urine and feam were cdbct+d from -me of tbe animab fa periods vary- ing from eight to 13 dam and d d v i t y Waa counted in pooled samplu. Extract8 were prepand from rabbit and moue liver to estimate incorporatian of the la- kl into body t h e &.

Pancremtic RNaae mcentratiarrs'did aol aornhte well with the r e p o d indiccs of utilizatim of exogcnou~ R N A (urinary a- c n t h and liver incorporatiaa d "C). Uri- nary acrrtion of "C wu) similar m the motme, rabbit. and cat, although tk Lt- tcr two apecia, have much lower levels of FtNasc. Mice had somewhat higher con- centration of "C in the liver. but this is not said to be a significant ditrerencc. To a p h i n this scsming contradictian. the author pro& poboible ention of RNase in diva . in other portions of the

bonudeaec m the pnata. Tbsbe pur-

~ C X X l t U h t V & h t h a p d @ O f M a n I - - f d ~ n i f ~ r m l y bbcid "GRNA to

184 N V I R m O N U V U W S [ Vol. 47, No. 6

intestine, oc by inteati~I microflora. Tbe amtent dRNm in t h e d i v a is known to b e d , and the inbstine and cecum at aome rpecim were investigated and f d to aacrete cmly negligibk amamta of the enzyme. Thc a u k that in n m - nuninart bcrt;ivaa, U e the rabbit and tbe borw, ing- are held up in the d M tive tmct h g enough for evm d mount. of the euyme to be effective in metabohzing the dictuy RNA. He abo aqgata that oopaphagy may be a factor ia tbae rpeciss with low level9 of pancrr- atice W, riacC it d d allow n'boau- h a beamd chna at the ~barsk.

AltCmatidy. n d WW by the author, the wide varintim in pan- .tic RNaae activity m y be irrelevant to utilization of nogenars R N A among the four opecia investigated. Tbc rate limit- ing factor d d well be acmething o t h r than the panmatic enzyme. In mpport of this antention, f e a l "C, presumably in- dicative of unabsorbed tmt done, since moat of it appeand within the first day. was 35 per cart oftbe dose in m e monkey, approximately 4 per ant in three rabbits and about 18 per ant in tbe three mice. Concentmtim of RNase in mouse pancreas b about Gve-hundredfold that of tbe m a - key m rabbit. bo h e r fadom are dnri- a rdy inftuarcing the proportion of exo. genolrs R N A digested and absorbed.

Bnruard propaam that the very hi& leveb of pancreatic Wane found m spe- cies poclsessing ruminant-like digestive proceaas, 1.200 rg. per gram in the cow ast compared with 25 pg. per gram in the horse and 1 rg. per gram in man, reflect the armrsity fa breakdown of RNA of the microbial population. Bacteria have a high RNA-nitrogen to total nitrogen ratio and a large proportion of the dietary nitrogen is incorporated into bacteria of the mmen. Abomasal digestion of the bacteria frees the R N A for digestion by pancreatic RNaae. In addition, R. C. Elliott and J. H.

TOQPS ( ~ 4 f i y ~ in, (rm)); TOPP and Elliott, h i d . 206, 4 s ( I S % ) ) have &own that excretioa of dmtoin and uric r i d in the urine ofrbeep and conr b COT- related with aucleic acid amantmtim d the rumen which. in turn, in dependent up<n dietary protein.

Wotphonm is present an 2 to 6per cant of the dry weight of tbe lumen microa- ganiama and the weight of phoqbrua in tbe lumen R N A i m .ban 6.S per cent of tbe weight of the m p t d nitrogen. This m w that, in order to maintain the mi- d i a l papuhtim, at l d 6.6 g. d phoa pborus per 100 g. of nitzagen muat be in- duded in the diet for microbial RNA h, m i n g nutilizatim of the two dement. b tbe amme. A d d i t i d require- ments for other phoqhte compound^ a d d double the amount necemuy. Bc- mule of tbe substantial requirement for p h c m p h in nminanta and becaw of the frequent inadequate amamta famd in heir diets, pbaepborus recycling is eaaea- tial. Bunard mamtaim that pancnatic RNaae and the phasphat.ses are necessary for this recyding and are nsparsible for the release of f i q h a t e in the intartine and stomach. Absabed phosphate is su-b aequently rca~ned to the lumen in dim which has beem ahown to contain relatively large amounts of phoephate. Also, tracer ntudies have been ubcd to demonstnrte subatantid phosphate sccntion directly through the rumen wall (A. H. Smith, M. Kleiber, A. L. Black, and C. F. Baxter. J. Nuhitian 57, 507 (1955)). The amount secreted may be five to ten times the d i e tary intake.

Barnerd cancludes that pancreatic RNase is eaaentinl only in those vertebrate spe- c i a With ruminant-like digestive procemes in that it makes available the phoephatg neceesary for the animal and its microbial population and also provides extra nit- gen under adverse nutritional conditions. He believes that the enzyme is of ~ n l v

SUGAR, CHROMIUM, AND SERUM CHOLESTEROL

l n d interest in trace elementa haa highlighted the fact that certain mi- outrients may indeed be critical in mam- malian nutrition. Of these, chromium ap pears to be emential in rats for a t le.ast me metabolic hrnctim and haa been strmgly implicated in additional sy&ems and spe- cies. w. Mertz (Fed. h. 26,186 (1967)) identified trivalent chromium aei a cofactor with insulin at tbe cellular level and aei a “glucaee tolerance factor” in rats and, poe- zi.b!;., 5 h.z=q being (& Nutrition Re- okws 25,49 (1m; & 235 ( I s % ) ) .

The involvemeot of chromium in lipid metabolism began with the work of C. L. Curraa (J. B i d . Chem. 210, 765 (19%)). in which he found that trivalent chromium enhanced hept ic synthesis of cholesterol and fatty acids in rats being fed a com- mercial laboratory ration. 'Ibis effect of chromium was further demonstrated by work showing that chromium supplemen- tation of a low chromium diet lowered Serum cholesterol levels in rats (H. A. Schroeder, W. J. Vinton, Jr., and J. J. B a h . Roc. SOC. Exp. Biof. Med. 109, 859 ( I S ? ) ) . Chromium supplements also tended to eliminate that age-linked in- crease in Semm cholesterol observed in male rats deficient in chromium.

parallel sbdiea uaiag human sub- have yielded fragmentary a d inmclu- give result^, an have the bulk of human studies linking chmmium to gtucose tder- mce and diabetcs mcIlitua. Nonethelea, the pamible nlevana of this work for major probkma in human nutritim and public heahb is provoartivc. Tbun, botb Me- (Fed. Boc. #, 186 ( 1 9 3 ) ) and Schroeder (Am. J. Clin. Nutnition 21, 230 (1-1 have recently offered the bypath- esis that atherosclerosis and mme forma of diabetes mellitus may rearlt, in part. from human chromium deficiency.

An attempt to bring animal esperimen- taticm M chromium somehow clarcr to the realm of human nutrition is apparent in the most recent work of Schroeder (J . Nutrition 97, 237 ( I = ) . In these erperi- ments male and female w e rats were maintained on a low chromium diet an- sisting of 50 per cent mmme. 30 per c a ~ t Toruh yeast. and 15 per cent lard. with added ealta and vitamins. b e n t i d trace metals were provided in dnnking water while environmental contamination was minimized.

Animals were grouped according to type of dietary sucmee: (1) white sugar contnining 0.02 p.p.m. chromium without